Seluyk is a Prothean-turned-Collector who broke free of the Reapers' grip over his mind. After regaining control, he joins up with Shepard in hopes of stopping the Reapers before he loses his mind again.
He is only obtainable if Javik is part of Shepard's squad.
Mass Effect 2
Seluyk is first encountered as an unnamed, masked Collector with notably spiny armor. He is little more than an enemy that gets knocked off a platform in one of the cutscenes on the Suicide Mission. It is not until the next game that he gets a name and a face.
Mass Effect 3
In Mass Effect 3, Seluyk is encountered when Shepard finishes with his first Citadel visit after acquiring Javik. Upon approaching Javik's quarters, Shepard will overhear a conversation between Javik and someone else. Upon further investigation, they will find the two conversing; Seluyk will be pleading with Javik for help with his inevitable indoctrination, while the latter will coldly state that he is a threat to everyone on the Normandy. They will eventually notice Shepard, who can either react aggressively or politely; the former will result in Seluyk holding his hands up and trying to explain his situation. The latter will achieve the same result, albeit more calm. Either way, Seluyk will remark on Shepard's ability to understand him and approach them to share memories of his capture during the Protheans' fall, butchery and reconstruction as a cyborg, as well as his regaining control of his body in the end of Mass Effect 2 and retrieving of his old, slightly damaged armor. He will insist that he is not a threat for the moment and offers his aid to Shepard. Four options are available: accepting Seluyk's proposal with either a friendly or hostile tone, or killing him for either the good of everyone else or the fact that he's a Collector.
Should Shepard allow Seluyk to be part of the Normandy's crew, Seluyk will take up residence in the Port Observation Deck, commenting on its lighting and positive atmosphere. Shepard will be able to ask him about his experiences and get him to open up about his indoctrination, where he will express his determination to stop the Reapers before he loses his mind again. Shepard may discuss the Avatars in an attempt to learn more about Protheans than Javik would let on; Seluyk will claim to represent willpower.
Seluyk is considerably more positive than Javik despite his torturous experience serving the Reapers. He will elaborate on this attitude, stating that it gives him the drive to push on and not give up. He still shares similar beliefs to Javik, however, such as a cold, imperialistic outlook on the galaxy. He possesses a callous but highly rational personality, viewing anyone who stands in the way of his goals as a threat that must be eliminated by any means necessary. Despite his views often causing discomfort among the Normandy's crew, he is sociable, using the bonds he forms with others to fuel his resolve and keep his indoctrination at bay. Likewise, the anger he harbors towards his enemies for harming said loved ones motivates him to seek vengeance and keep fighting.
The crew frequently reference Seluyk's unusual appearance, comparing him to the Collectors and noting the eight disturbing claw-like limbs on his back (which he used to hold onto weapons and other items) and mechanical teeth. He typically responds that the implants were the only good things the Reapers did for him, though he is less fond of the extended metal claws protruding from his right fingers. Garrus will proudly say that Seluyk is the one thing uglier than him if asked about him by Shepard. Crewmembers often ask him about his experiences, prompting horrendous stories about the previous cycle's downfall or the few memories he retains of his existence as a Collector. James and Garrus frequently contact Seluyk over the comms to compare achievements and war stories, though Seluyk retains a more grim outlook on war due to the nightmares of the Prothean-Reaper War.
If brought to Sur'Kesh, Seluyk makes known his sympathy for the krogan, understanding the feeling of being part of a virtually dead race. However, he mostly views the curing of the genophage as a simple means to an end—with the krogan able to reproduce more effectively, the offspring will be effective soldiers in the war against the Reapers. If Wrex is alive and leading Clan Urdnot, and both Javik and Seluyk are in the squad, he will jokingly suggest that one of them get a sex change if they want to bring back their race. If Wreav is the leader instead, Seluyk will make note of his being a Collector and casually suggest experimenting on the krogan; this shuts him up and ends his arrogant comments for the time.
Seluyk expresses approval towards the turians when he learns of the bomb they placed on Tuchunka, viewing it as a necessary precaution to keep the highly-aggressive krogan in check. As such, he feels the bomb should be kept in place, but acknowledges that disarming it is the only way to progress krogan cooperation in the fight against the Reapers. After the bomb is defused, he will be somewhat saddened by Victus's death, but the lieutenant's poor leadership skills and Seluyk's own experience with loss leaves him mostly indifferent.
He and Javik both find the return of the rachni chilling and can be found conversing about them. They will elaborate on this if approached by Shepard, explaining that they bred the rachni into what they are. Seluyk will strongly favor making peace with the Rachni Queen on Utukku—though if the original queen was killed and replaced by an artificial one, he will sense treachery and attempt to protest saving it. Either way, he will begin to extensively research the rachni after the mission; if the original queen from Noveria was saved, he will look forward to the species' future. If she was abandoned, he will be obviously disappointed, adding it to his already long list of disagreements with the current cycle. Sparing the artificial queen will result in a similarly frustrated reaction, whereas leaving it to die it will be met with praise.
When the issue with the genophage is brought back up, Seluyk and Javik will have similar stances. Both agree that deceiving the krogan would be beneficial as it would secure alliances with them and the salarian dalatrass. However, Seluyk, if brought to Sur'Kesh, will note that he could sense sympathy among several of the salarians there and ties it to Wrex—assuming he was spared in Mass Effect. He also sees an alternative method: intimidating the dalatrass into realizing what her refusal to help is risking. He begins to voice ideas of what to do to the dalatrass after the war, hoping that she will be killed for endangering the galaxy.
Seluyk will make few comments during the mission to Grissom Academy. However, he does make it known that he values the biotic students, considering their abilities most helpful to the war effort. If Jack is present, she'll express distrust of Seluyk; if Javik is part of the squad, she—like Wrex—will sarcastically suggest a sex change, much to the Protheans' agitation. If Jack did not survive the Suicide Mission, Jason Prangley will take charge and Seluyk comments on his leadership potential, sensing his past as a shy, quiet young boy. He, like Javik, will note that during the Prothean-Reaper War, there was no time for the Protheans to educate their children like the Academy. He thus considers this a chance to boost his determination, hoping to see the achievements the biotic students will make.
Seluyk's hatred for Cerberus intensifies when he learns of the coup on the Citadel. He frequently comments on Cerberus's actions throughout the mission, furious that they would divide and weaken the cause against the Reapers for a chance to gain influence. To him, Cerberus should have been contributing to the war effort until now, now seeing them as threats that need to be eliminated as soon as possible. His anger is further fueled when he discovers that Councilor Udina was behind the coup, mentioning that he wishes to dismember him. Javik can elaborate on this after the mission: in the times of the Protheans, traitors would have their limbs ripped off and be forced to eat their own flesh or starve. However, he sympathizes with Udina in a way; the Citadel was offering very little support, and Udina had been driven to desperation. If Ashley/Kaidan ended up getting in the way and killed, Seluyk will shrug it off; had they been reasonable, he would've provided the truth for them via touch.
He is relatively unmoved by the horrors encountered at the Ardat-Yakshi monastery, comparing the situation to some of the twisted monsters he faced in the Prothean cycle. However, he will shudder as he begins to go into detail and changes the topic. He will then resume staring out the window of the Port Observation Deck, half-listening to the conversation between Javik, Garrus, and Ensign Copeland and half in his thoughts.
Seluyk will show a rare opinion among the Normandy's crew when the issue with the geth is brought up. He appears to understand that the ones worshipping the Reapers are not the same as the mainstream geth. Furthermore, he views them as possible useful allies, strongly urging that Shepard seek peace with them. Throughout the missions involving the geth and quarian conflict, Seluyk can be seen debating Javik on the struggle, insisting that the geth are not the same as the ones the Protheans faced in their time. After Legion or the Geth VI is rescued, Seluyk will add more points to his argument, pointing out that the quarians restarted the fighting and drove the geth into the arms of the Reapers. He'll say that the geth are a peaceful race that can be persuaded to join Shepard's cause—the quarians just need to listen to reason. He'll take this dispute to the War Room to argue with the quarian leaders later on, hoping to convince them to make peace with the geth.
Should Shepard choose to side with the quarians, Seluyk will express frustration at the decision. He will assert that he trusts Shepard's judgment, but still sees the quarians as the ones to blame for the conflict. Seluyk will make note of Shepard's visions in the Geth Consensus—having sensed them—and uses this to back up his argument. If Shepard is in a relationship with Tali, Seluyk will mention that as well, suggesting that he was motivated by emotion over logic. In the end, Shepard will dismiss him, ending the conversation.
If Shepard sides with the geth instead, Seluyk will still be displeased, agreeing that the quarians brought their fate upon themselves, but still wishing for the races to have made peace instead of one being wiped out. He and Shepard will once again argue about the decision (much like if Shepard sided with the quarians, albeit with dialogue differences) that slowly becomes heated until the Commander drops it.
If peace between the races was reached, Seluyk will give Shepard his utmost approval, seeing the event as proof that organics and machines can coexist. He'll wonder how Javik will react and contemplate bragging to him that he was right.
During the mission to Thessia, Seluyk may show up unexpectedly at the Temple of Athame if Javik is part of the squad and his relationship with Shepard has been positive (regular conversations, Rachni Queen being saved, peace between geth and quarians) and vice versa. He and Javik will comment on the various artifacts in the temple, slowly dismantling Liara's religious beliefs; they will point out the Prothean involvement in asari development, much to her agitation. He'll later comment on Liara's outburst towards Javik on the Normandy post-mission, saying that the whole floor could hear the argument.
If Shepard did nothing to end the argument, Seluyk will strongly suggest to not have Javik and Liara near each other; the former will only be annoyed by her presence, and the latter may become unstable.
Alternatively, Shepard can pacify the incident themselves, for which Seluyk will thank them. He'll sense a bond finally developing between the two, albeit more on Liara's side than Javik's.
After the events of Horizon, Seluyk can be talked to. He'll compare Henry Lawson's experiments to numerous other attempts by races in the Prothean-Reaper War, always ending each story bluntly about how they failed. He'll see the atrocities on Horizon as further proof that Cerberus's goals of controlling the Reapers cannot be allowed to succeed; it is simply too much power in the hands of a madman. He'll strongly urge Shepard to destroy the Reapers by any means necessary before resuming his rest on the couch. If Tali is still alive and drunk in the room, he'll begin muttering to Shepard, asking them to please shut her up. He'll also make a comment if Shepard was in a relationship with Miranda, assuming she dies on Horizon, mentioning his own losses of many loved ones.
Seluyk will be greatly relieved after the assault on the Cerberus base, believing the separatist faction to be in ruins and Kai Leng to have been a grave threat. He will admit to being disappointed that the Illusive Man escaped, but remains confident that he will be dealt with in time.
By the time of the final battle, Seluyk's survival depends on a variety of Shepard's actions. He'll be unavailable as a squadmate for the first half of the mission and instead be trying to motivate the crew of the Normandy before joining the fight himself. When Shepard reaches the London FOB, Seluyk himself will begin making his way there. At this point, Shepard's interactions with Seluyk—as well as the galactic readiness level—will determine whether he makes it. If Shepard had minimal or mostly negative relations with him and the galactic readiness is low, the squad of troops accompanying Seluyk will be wiped out and he will have to fight off the Reaper forces himself. In a short cutscene, he'll start losing his weapons to the surrounding enemies and attack the crowd with his knife until he is overtaken and pushed off a ledge.
On the other hand, if there were enough war assets to keep Seluyk alive, he'll reach the FOB. This time, his fate depends on Shepard's relationship with him; if the Commander was aggressive or cold to him, or otherwise weakened the Prothean's resolve by making him distasteful towards the current cycle (siding with the quarians instead of peace, going full Renegade, etc.), he will have very little motivation to continue on. He'll have developed a bleak outlook and begins to prefer dying in the finale of the war instead of surviving and putting up with the rest of the cycle. At this point, he succumbs to his indoctrination and lashes out at Shepard, resulting in a fight composed of Paragon/Renegade interrupts that ends with Seluyk's death-by-bullet. Javik will remark on the incident, not surprised in the slightest.
A third possibility involves a moderate to positive or even romantic relationship (assuming Shepard is female) with Seluyk, on the condition he survives making it to the FOB. He can be conversed with on multiple topics, such as his plans for after the war or his hopes that he'll finally be freed of the Reapers' influence on him. If Shepard is in a relationship with Seluyk, he'll look forward to spending time with her in times of peace. He may relate this to Javik and Liara if Shepard intervened in the post-Thessia argument, hinting at some possible chemistry.
Assuming he survived and Shepard activated the Crucible, he'll be seen in the end with the rest of the Normandy crew, paying his respects to the Commander. If Shepard chose to control the Reapers, he'll be surprised that it actually worked. A Synthesis ending shows a brief clip of Seluyk laughing at Javik.
Seluyk can be killed in the final push towards the Conduit if Shepard did not gather enough of the galaxy's military forces.
Mass Effect 3: Leviathan
Seluyk will become very interested in the Leviathan when he hears of it betraying the Reapers. He'll compare this to his own defection from them and wonder if the Reapers really do control their own actions.
When it is unveiled that the Leviathans were actually the creators of the Reapers, he'll become skeptical of the aliens and urge Shepard to do the same. Seluyk will back this up with the creatures being the first apex race of the galaxy and their ability to indoctrinate, fearing that they may become just as big of a threat as the Reapers if the galaxy lets its guard down.
When the Leviathans use their own powers of enthrallment to sever most of the remaining Collectors from the Reapers' control, Seluyk becomes grateful to see the remainders of his people (besides Javik) free and fighting for the rest of the galaxy. He hopes to some day meet these "Awakened Collector Adepts", as they become known as, and remind them of the Prothean Empire. Though he's happy that the Leviathans freed the Collectors, he maintains his distrust if spoken to about it. With the Collectors now serving in the war effort, he also regains access to their technology; in combat, he can use Seeker Swarms against enemies.
Mass Effect 3: Citadel
While Shepard is on shore leave, Seluyk will greet them and be thankful for the chance to relax, though he continues to worry about the war due to his own experiences. After the shore leave is interrupted by mercenaries, he'll join in on Shepard's conversation with one of their squadmates over the comm channel; if he and Sheaprd are in a relationship, he'll express great concern and assure her that he's coming as fast as he can. Likewise, if Seluyk is Shepard's love interest, they'll encounter him taking out several mercenaries before becoming a squadmate. He'll also express a degree of excitement, having hoped for a nice, small fight.
During the meeting on what to do next, Seluyk's dialogue varies. If he was Shepard's love interest, he'll simply be happy she's still alive. Otherwise, he simply compliment's Shepard's combat prowess, considering they just took out a large amount of mercenaries with a pistol, unarmored. Depending on how friendly he and Shepard are, he may also joke that he never sees them fight that well with better equipment, and suggests going into combat like this more often.
Ultimately, by the end of the DLC's story, Seluyk will express disgust with Brooks and the Shepard clone, believing their plans to kill Shepard only wasted resources that could've been used against the Reapers. He states that Shepard is the most valuable asset to the war effort, and that losing them to a mentally-unstable clone would've cost the galaxy in the long run. Likewise, he views imprisoning Brooks as a waste of money, food, and other resources, believing that everything should be dedicated to stopping the Reapers; as far as he's concerned, prison is too soft a punishment for what she did. He is clearly eager to show this to Shepard, as he frequently discusses with them throughout the story what he thinks should be done. His reaction to what Shepard does with Brooks will depend on the outcome; imprisoning her will, obviously, result in criticism. If events lead up to Brooks being killed, he'll either express relief if Shepard does it, or joy if he gets to do it.