For characters seen throughout the series, such as the Big Daddies, Little Sisters and Splicers, please see here.For characters originated from the first game, please see here.
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The protagonist of BioShock 2 and the Player Character, a Super Prototype Big Daddy and the first to be successfully bonded to a Little Sister, who happened to be Eleanor Lamb. Unfortunately for Delta, Eleanor's mother Sofia managed to recover her and used an Hypnotize plasmid to force him to shoot himself in the head. Delta was revived years later by Doctor Tenenmbaum to stop Sofia who has taken over Rapture and aims to to something terrible to Eleanor.
Made of Iron: Subject Delta endures a lot, even by the standards of most Alpha Series. In the cutscenes alone, he's hit by a wall of high-pressure water when a Big Sister shatters a bay window; he's trapped in a room and pelted with gunfire and molotov cocktails until the floorboards under him break, sending him on a three-story fall; the train car he's riding gets torpedoed by Father Wales; Sofia Lamb floods Siren Alley, nearly crushing him to death when a metal door's slammed into him by the resulting water pressure; his link to Eleanor is disrupted when Sofia half-suffocates her with a pillow, leaving him on the brink of death; a corridor of demolition charges explode in his face, and he has to ride to the surface while hanging onto the side of Sinclair's escape pod, being battered and crushed by intense pressure on the way.
Magic Knight: Optional. You can use just your drill and plasmids in this game, and there's a tonic which (optionally) forces you to use basically nothing else. With the right tonics and plasmids, this can be incredibly effective.
Papa Wolf: He's a Big Daddy. It's literally part of the job description. Even more so on account of the fact that he is bonded to Eleanor alone, and will tear through an entire city's worth of Splicers, Big Daddies and Big Sisters to get her back.
Roaring Rampage of Revenge: It's entirely feasible to do a playthrough where you murder every living thing between you and Eleanor, including those who don't really deserve it and/or have no way of defending themselves.
Spirit Advisor: In most of the endings, Eleanor injects herself with Delta's ADAM, allowing him to live on inside her mind.
Super Prototype: He's more mobile than a normal Big Daddy and can use plasmids, at the cost of not being quite as tough. Dr. Suchong also mentions that he's superior to what is to come, but they had to Nerf the following versions to the point of "mediocrity", mostly because they would be too dangerously overpowered. Not to mention too expensive.
Some of Gil Alexander's diaries also note that the "one Sister, one Daddy" linking system built into Delta ultimately caused more problems than it solved. Given the game's plot this was no doubt very true.
Tomato in the Mirror: Not including the "Was Once a Man" bit below (which isn't surprising since all Big Daddies were made from people), Sofia Lamb actually brings this up in conversation, talking about how cruel it is to force awareness onto something that was human but is no longer.
Was Once a Man: Apparently, he was known as Johnny Topside, a deep-sea explorer who found Rapture whilst exploring the sea-bed. At least that's what Stanley says.
Dr. Sofia Lamb
"Until ADAM, the human animal was a slave to the gene... the inner Tyrant. Lust... greed... rage; self obsession was etched into our DNA. We were not pulling the 'Great Chain of progress,' as Ryan believed - but shackled to it. But now, in theory, we can redraft the human blueprint. Serving the common interest can become as natural as breathing."
The Big Bad of BioShock 2. Doctor Who fans may recognize her asAgatha Christie, played by Fenella Woolgar. An idealist as obsessive as Ryan, but with a philosophy that's diametrically opposite. She believes in absolute collectivism, denouncing individuality and even sentience as a curse. She plans to use her daughter Eleanor to set the foundations for Utopia, whatever it takes.
Abusive Parent: From the opening cutscene, it's plainly obvious that Sofia cares nothing for Eleanor, only for how she can be used to further her own plans. Of particular note is the way she says "[Eleanor] is mine," the emphasis treating Eleanor more like an object than a person.
All-Loving Hero: She's anything but, yet still manages to be unique deconstruction of the idea. She loves everyone without bias. Or, in other words, no one person is important to her in any way, and everyone is a potential sacrifice for the sake of her Utopia.
Berserk Button: She couldn't stand Eleanor rejecting her ideology and went to increasingly erratic and extreme lengths to stop Subject Delta. Finally she smothers Eleanor in a last ditch attempt to stop Delta; she had no way of knowing this wouldn't kill Eleanor as well, but it's quite clear she didn't care.
Break the Haughty: Suffers from this in the semi-good ending of the game, especially when Eleanor makes it clear that she only saved Sofia's life to make sure that she'd go on living in the knowledge that her Utopia had failed and her daughter had betrayed her.
Dirty Communists: While never identified as one per se, Ryan remarks the very presence of her collectivist ideas will cause another Kremlin to spring up in Rapture. He was right... except if he hadn't betrayed his principles and thrown her in prison with the rest of his enemies just as she planned, she'd never have been able to build her army. Just like how his anti-smuggling laws created Fontaine.
Good Smoking, Evil Smoking: Chain-smokes cigarettes—apparently as a result of stress, as the only time she's seen doing so is when Delta is standing right outside her door.
Hypocrite: She preaches collectivism, but she is practically as possessive as Ryan himself. This was true even before she came to power, as she leaves the surface because she believes others have corrupted HER collectivist ideas. She also continually berates Delta for stealing her child away, yet has been kidnapping children from the surface to make them into Little Sisters.
I Have No Daughter: Right before smothering Eleanor, she bemoans that Eleanor has been learning from Delta's actions instead of her own, rendering her little better than him in Sofia's eyes.
Lack of Empathy: Gil Alexander explains that Sofia does have empathy, but for everyone equally. The net result is that she doesn't give a single damn for any one individual, only caring about how they can benefit her cause for humanity as a whole.
Manipulative Bitch: She engineered her own jail conviction to gain popular support as a martyr and place herself in a facility far from Rapture where she could turn the inmates into an army. Even Fontaine is impressed.
Moral Sociopathy: A curious case in that she probably did not began as this, but by the time of BioShock 2 she has definitely become a sociopath obsessed with her philosophy.
Nothing Personal: She goes as far as to say she loves Subject Delta just as much as anyone else, before sending a pack of splicers to kill him. As noted above, she "loves" everyone, but to such an insignificant degree that their lives mean nothing to her.
Not So Different: Despite being Ryan's Foil, they are incredibly similar. They are both hypocrites willing to bend their philosophies to their own ends. They also both treat Rapture as a means to an end, more concerned with their imagined utopias than the actual city or inhabitants under their control. And as other trope entries on this page show, the similarities do not stop there.
Psychic-Assisted Suicide: Attempts to do this with Subject Delta at the beginning of the game by using a mind control plasmid. Fortunately, he gets better.
Well-Intentioned Extremist: But not in the way most would think: her actions and audio diaries quite clearly reveal she never had any intention of saving Rapture. Her real goal is to create a "Utopian," who would be able to serve the common good with no concern for their own desires, and gradually convert the rest of the world into similar beings.
Augustus Sinclair / Subject Omega
"You do business as long as I have, and you learn to pick a brand name from the writin' on the wall."
Action Survivor: Lives through the Rapture Civil War and ten years of Lamb's rule of Rapture while remaining unspliced. Unfortunately for him, his skills failed him when he tried to rescue Delta and Eleanor.
Affably Evil: Amiable and without scruples from the word "go."
An Economy Is You: His principal business enterprise is a twofold scheme that virtually guarantees repeat business - his Sinclair Deluxe apartments cater to poor and miserable individuals by offering cheap rooms at low prices, and the "tenant discount" available at his Sinclair Spirits bars makes sure said tenants remain poor and miserable.
Anti-Villain: Type I: he was involved in a lot of the more disturbing experiments and schemes run in Rapture, including the development of the Big Daddies, but he had several lines he refused to cross and genuinely wants Delta to succeed in his mission to rescue Eleanor- especially since it will mean he gains access to technology he can sell on the surface.
In a sense he's what Frank Fontaine would be like if he had some shred of morality and weren't a sleazy conman.
Apologetic Attacker: Gets turned into an Alpha series Big Daddy near the end of the game and forced to fight Delta, desperately apologizing to him and Eleanor along the way.
Big Damn Heroes: Attempts one of these to free Delta from captivity. It doesn't go well for him.
Boss in Mook Clothing: Sinclair as an Alpha is considerably tougher than the ones you've fought so far, probably owing to his recent conversion and Lamb's meddling.
Consummate Liar: Subverted. Despite having similarities to Frank Fontaine, never does he actually lie to Delta. In fact, he's pretty much one of the most honest folk in Rapture and a foil of sorts to everything Fontaine represented. Which doesn't save him from what Lamb has in mind.
Freudian Excuse: Apparently, his grandfather was a strong believer in working in the name of "The People," and "The World Entire," and drowned in the construction of the Panama Canal- thus giving Augustus Sinclair an early bias against altruism.
Functional Addict: He's apparently indulged in ADAM-based plastic surgery, but evidently not enough to drive him to Splicerdom.
Genre Savvy: Much like Fontaine, Sinclair's successes (by his own admission) can be measured by his ability to predict Ryan's failures. Just as Fontaine rose to power by selling the things Ryan tried to deny his citizens, Sinclair became successful by banking on Ryan's inevitable failure to maintain his vision and a need to remove undesirables from his city.
Good Smoking, Evil Smoking: Can be seen holding a cigarette holder on the one occasion Delta meets him face to face. In defiance of convention, however, Sinclair's not truly evil, just mercenary.
Sofia Lamb's daughter and Delta's former Little Sister. Constantly in sedation, as part of Lamb's master plan. Voiced by Sara Bolger and Sydney Unseth (as a child).
Acquired Poison Immunity: Apparently due to her exposure to large amounts of ADAM as a child, Eleanor can tolerate extensive splicing without any of the side-effects. Unfortunately, this makes her the perfect candidate for her mother's experiment...
Action Girl: Even among other plasmid wielding citizens, Eleanor's talents are formidable. Her time as a little sister has also made her immune to the negative side-effects of splicing. Even more impressive once she becomes a Big Sister.
Badass Bookworm: Extremely intelligent and knowledgeable as part of her upbringing.
Badass in Distress: Turns out she's very capable of looking after herself. It seems her mother was less afraid of others harming her and more afraid of her daughter simply going her own way.
Bratty Teenage Daughter: Sofia laments the fact Delta is turning her into this. However in reality she's a subversion as she's actually very respectful of her father for his past kindness and very much justified in acting out against her mothers attempts to crush her sense of identity. In the good ending she even saves her mother despite the deserved animosity, meaning she rose above her so called childish antics.
Child Prodigy: Highly intelligent even in her earliest years, dismantling electronic devices and reassembling them for fun. She also had a tendency to disable her mother's security systems so she could sneak out of the house (not that this approach always worked).
Teen Genius: Masterminded the plan to resurrect Subject Delta.
Corrupt the Cutie: As Eleanor is watching virtually everything you do, harvesting the Little Sisters and killing defenceless characters will result in her becoming just as ruthless and violent.
Curb-Stomp Battle: When you summon Eleanor to fight with you in Persephone, she pretty much stomps all over the splicers.
From Nobody to Nightmare: From a little sister to basically an invincible, teleporting Big Sister with telekinesis. Even more of a case in the Bad ending, in which case she's a Blood Knight that derives joy from both killing things and watching them die, and has both her own genius and Delta's cunning and experience on top of all those powers.
Evil Brit: Becomes one in the "bad" endings. Otherwise, she presents an Aversion of this trope, especially when compared to the examples provided by her mother and Dr Alexander.
"October 9th, 1967. Hello. My name is Gilbert Alexander, and by the time you hear this, I will be clinically insane."
Another one of Fontaine Futuristics' leading scientists, Gilbert "Gil" Alexander replaced Dr. Suchong as head of the Protector Program; in this position, he created the Alpha Series and the symbiotic bond with their Little Sisters—making him indirectly responsible for Johnny Topside's conversion into Subject Delta. Following the collapse of Rapture's security police, he also designed the city's automated security system. However, he eventually fell in with Sofia Lamb and the Rapture Family, assisting in their development of a "Utopian," even submitting himself to the first attempt to create one. Unfortunately, the experiment resulted in him being horribly mutated and deranged by mass amounts of ADAM, ultimately reducing him to a nightmarish aquatic monster kept within a tank in the basement of Fontaine Futuristics.
The Atoner: Dr. Alexander was genuinely ashamed that he'd been responsible for Eleanor's transformation into a Little Sister, and worked for many years to undo the conditioning he'd forced on her.
Ax-Crazy: He has been known to "fire" Splicers under his employ, carving up "You're Fired" above and around them, and hang them on the walls when he's not sending them after Delta. He literally makes every other insane person in Rapture (i.e just about everyone else) look tame by comparison.
Bad Boss: Following his descent into insanity, "Alex the Great" has assumed total control of Fontaine Futuristics and declared himself CEO; given that there's no way of creating new products and no paying customers anyway, most of his time is spent torturing his splicer "employees."
Berserk Button: Never, ever call him "Mister A" (or anything other than "Alex the Great"). Some poor Splicer under his "employ" found that out the hard way.
Body Horror: We don't see his transformation take place, but given that the monster inside the tank looks like a cross between a fetus and a squid, it wasn't pretty.
Crazy-Prepared: He might have been undergoing a horrific mutation and almost half-insane at the time, but Dr. Alexander was very careful in setting up a way for Delta to Mercy Kill him; first, he left several pre-recorded messages throughout the Fontaine Futuristics building, containing useful advice and essential passwords to restricted areas; secondly, he planted a cache of weapons and security bots to assist anyone who might come to the rescue; finally, just to make sure his rescuers would be sane enough to outwit his insane alter-ego, he programmed a bio-scan to exclude any visitors who weren't mentally up to the job.
Deadpan Snarker: "Congratulations, you've won a battle of wits with a home appliance."
Evil Brit: Apparently English—or at least he speaks in a Received Pronunciation accent- but not actually a villain until massive doses of ADAM administered by Sofia Lamb drive him insane.
Flunky Boss: Controls everything in Fontaine Futuristics and gleefully flings everything he has at you, but once you've taken out all the mooks, he's practically helpless since he can't get out of the tank to fight you.
Gadgeteer Genius: Not only did he mastermind Rapture's security systems, but after his mutation began he also modified the branches of it around Fontaine Futuristics to provide help to anyone answering his Distress Call. After his mutation drove him insane, he also equipped numerous security drones with monitors and speakers to carry his voice and face- part of it, at least- to his "employees."
Hopeless Suitor: One of his audio diaries indicate that he has something of a crush on Sofia Lamb- even referring to her as a "Secular Saint"- though he admits that her Love You and Everybody philosophy means that he can't be given preferential treatment.
Large Ham: In every single audio diary in the game, he's heard speaking as quietly and carefully as possible, so it's a bit of a shock to hear him start hamming it up as Alex the Great, chewing the scenery to a pulp and spitting it at Delta.
Mad Scientist: As one of Rapture's scientific elite, this is a given.
The Man Behind the Curtain: When Delta finally meets Dr. Alexander's horribly mutated self, he's not much of a threat without the splicers he employs; true, he may be a gigantic monster with the combined powers of hundreds of different splicers implanted into his body, but his reliance on his tank prevents him from physically attacking, and the bright light means that he can't even see Delta clearly enough to fight back in any other way.
Meaningful Name: The man's nickname is "Gill," and he ends up as an obvious aquatic lifeform.
Mercy Kill: His lucid self requests this from you, while his insane self begs to be spared; the decision's up to you.
Sanity Slippage: By the time he recorded the messages, he was in the latter stages of this.
Shock and Awe: His personal security drone can zap things with electricity. He uses it to fry "disobedient" splicers and carve messages into the walls, as well as open doors occasionally.
Shrinking Violet: In most of the footage taken of him, Dr. Alexander comes across as nervous and painfully shy prior to Lamb's experiments: in his audio diaries, he speaks in a halting, shaky voice, sometimes barely louder than a whisper; in the "Rapture's Best and Brightest" photo, he's at the very back of the group, looking as uncomfortable and agitated as possible.
Under Statement: Whenever he sends something at you it's said as normal workplace dialogue. This includes stuff like telling you he's 'sending an old co-worker of yours who wants to meet up with you'. It's actually the first Alpha you meet, and it's pissed.
The Voice: Subject Delta never sees Gil Alexander's entire face at any point in the game except in old photographs. Unless you're playing the PC version of the game, and you're prepared to use noclip.
Weakened by the Light: Dr. Alexander's new body can't stand bright light, and naturally recoils when Delta switches on the overhead lights in his chamber.
Stanley Poole was a reporter for the Rapture Tribune newspaper. He was hired by Augustus Sinclair on the behalf of Andrew Ryan to infiltrate the Rapture Family, and gather evidence so that Sofia Lamb could be imprisoned. It all went downhill for him from there.
Cower Power Poole spends most of his only level hiding in a booth while forcing Delta to clear up his dirty business.
Dirty Coward: Poole's only real concern is keeping his ass out of the fire, and he's willing to go to the most repulsive lengths to ensure that nothing bad happens to him: this includes having Eleanor Lamb kidnapped and converted into a Little Sister, sabotaging Dionysus Park so that the entire population drowned, and ordering Delta to harvest any Little Sisters that might discover what he'd done. And when things start getting out of control, he quickly lapses into spineless begging.
Evil Uncle: When Sofia Lamb was arrested, she ordered Stanley Poole and Grace Holloway to take care of Eleanor; as such, Poole ended up being called "Uncle Stanley," and proved his credentials in the evil uncle field by having Eleanor kidnapped to prevent her from ratting him out.
Functional Addict: Flashbacks show that Poole has been splicing, but he's obviously been able to keep his habit under control for now.
The Hedonist: Upon Lamb's incarceration, Stanley assumed stewardship of Dionysus Park, and apparently made the place live up to its name by hosting wild parties every night (on her dime, of course).
Hero-Worshipper: One of his audio diaries reveals that he's come to admire Subject Delta- or, as he was known when he was still a human being, Johnny Topside- after interviewing him for the Tribune. Poole also went on to frame him as a spy and have him imprisoned in Persephone, where he was transformed into a Big Daddy, so he obviously didn't admire Topside that much.
Oh Crap: At the end of his level, Poole suffers a doozy of an Oh Crap moment when Sofia Lamb remotely opens the door to his booth and informs Delta just who was really responsible for his conversion into a Big Daddy.
Paid Harem: One of the flashbacks shows Poole during one of his parties, which he financed by using all of Lambs money, and he has two girls next to him, obviously both slightly spliced, and he appears at least as he hopes to have his way with them both.
A lounge singer who was blacklisted by Andrew Ryan after singing protest songs against his rule, with her lover James being taken away. She became one of Sofia's most devoted followers, and was entrusted with keeping Eleanor safe when Sofia was imprisoned by Ryan. She failed in her duty when Stanley Poole betrayed Sofia and kidnapped Eleanor. Grace eventually saw Eleanor as a Little Sister being guarded by Delta, who broke her jaw when she tried to hug Eleanor, prompting Grace to believe that Delta was the one who kidnapped Eleanor and transformed her. Elenor gave her control over Pauper's Drop after taking over Rapture. Voiced by Sheryl Lee Ralph.
Badass Boast: "I remember you, monster. You stole Eleanor from me...twisted that baby girl into a thing so sick it can't even die... and now you come swanning into my neighborhood lookin' for me? Wrong turn, Tin Daddy. When we hang you from a streetlight, and you're choking out your last... I want you to remember my face."
Heel Realization: If Delta spares her, she realizes that he is not a monster and provides him supplies to stop Sofia and rescue Eleanor.
It's Personal: She isn't just following orders when it comes to hunting Delta. She was tasked with caring for Eleanor and got a broken jaw for her trouble when she tried to hug her in Delta's presence after she was kidnapped and converted. She believes Delta was the kidnapper, too.
Parental Substitute: Eleanor went to live with her after Sofia's incarceration, and it's clear that Grace's love for the little girl was far more genuine than her mother's.
One of the architects that Andrew Ryan hired to design Rapture alongside his brother Daniel, he was struck hard by the depression that hit the city and blamed himself for leaks in the city that killed citizens. He found comfort in Sofia Lamb's teachings, and restyled himself as "Father Wales", spreading her message as the leader of a religious cult called the "Rapture Family" in Siren Alley. Voiced by Richard Poe.
Simon's brother and his former partner in designing Rapture, Daniel felt none of the guilt his brother did when leaks killed several people around Rapture. He refused to follow his brother into Sofia's cult, and instead established himself as the head of the Pink Pearl Brothel in Siren Ally. Voiced by Graham Rowat.
The protagonist of the "Something in The Sea" Alternate Reality Game who was incorporated into BioShock 2 by fan demand. Mark G. Meltzer was an ordinary middle aged American citizen who developed an interest in a series of kidnappings of children along European coastlines, which unknown to him were being perpetrated by Big Sisters. Unfortunately, during his investigation, one Big Sister managed to follow him and kidnap his own daughter Cindy. He then dedicated the rest of his life to getting Cindy back, becoming one of the few Muggles aware of the existence of Rapture and one of fewer that managed to travel there. By the time Delta is revived, Mark is shortly ahead of him, searching for Cindy.
Badass Normal: He is unspliced, while in Rapture he only carries a pistol, and when the Big Sister captures him he still manages to get some shots off despite being distracted by his daughter.
Bittersweet Ending: He is captured by Sofia Lamb and is given the choice between death or becoming a Big Daddy and protecting Cindy. He chooses the latter. The player most likely kills him but can rescue Cindy. Or harvest her.
Tomato in the Mirror: Inverted. It is revealed at the end of Minerva's Den that Subject Sigma is C.M. Porter; C.M. Porter who has been assisting him turns out to be The Thinker.
Tragic Monster: Subverted in the end where he is back to an unspliced state.
Charles Millton Porter
A brilliant mathematician, Porter worked for the allies in London during World War II, losing his wife Pearl to the German bombings. He accepted Andrew Ryan's offer to work in Rapture, building The Thinker, the ultimate processing machine, which he hoped to use to resurrect his dead wife. He was betrayed by his partner Reed Wahl and imprisoned in Persephone, but eventually escaped and teamed up with Dr. Tenenbaum and Subject Sigma to liberate The Thinker and flee Rapture. Subject Sigma is eventually revealed to be Porter, with the one guiding the player actually being The Thinker. Voiced by Carl Lumbly.
Beware the Nice Ones: He's able to see through not only Ryan and Fontaine's schemes but also smart enough to stay out of Lamb's way upon guessing more or less what she's really up to. That's not even getting to his actions as Subject Sigma.
Necromantic: His main motivation for humanizing the Thinker was to bring his dead wife Pearl back to some degree with the Thinker's advanced approximation of Pearl's personality. Although the Thinker is successful in its emulation, Porter realizes that it isn't Pearl and stops.
Nice Guy: He's shown through the audio logs to be one of the nicest, most sympathetic people in Rapture. And also one of the most sane.
Only Sane Man: While he does have some deep issues involving the Thinker and his dead wife, the fact that he doesn't fall for Lamb's "solution" says a lot about his state of mind.
We Used to Be Friends: Him and Wahl. He states that he doesn't care if Wahl makes it back to the surface or not, and would prefer him to rot in Rapture.
The antagonist of the Minerva's Den DLC, Porter's former partner who betrayed him to take control of The Thinker alone, believing that it can be used to crack a perfect equation that can predict everything. Voiced by Keith Szarabajka.
Authority Equals Asskicking: For a skinny mathematician, Wahl is remarkably strong even by Splicer standards when you finally fight him, as he can take almost as much damage as a Big Sister (though, since he lacks Big Daddy damage resistance and effect immunity, it's pretty easy to juggle him to death with your plasmids and weapons).
Foreshadowing: After realizing that Sigma is Porter, Wahl hints to Sigma that he knows it, by pointing out the inconsistencies of Porter suddenly returning from Persephone, the Hell Hole Prison over them all, and completely sane and normal.
The Last of These Is Not Like the Others: Unlike the other main antagonists of Bioshock 1 and 2, Wahl's radio ambience shares the generic kind that Atlas, Sinclair, etc. use, rather than the deep ominous one used by Ryan, Fontaine, and Lamb.
Mad Mathematician: Minerva's Den is full of equations scrawled on the walls in chalk, paint and even blood, all presumably Wahl's work in his efforts to find the "predictive equation", an equation he believes can be used to predict the future.
Nice Hat: While not a hat per se, the headgear he wears is intriguing.
Sweet Tooth: The floor to his office is littered with bottles of Hop-Up Cola, implying he's been using the drink's high amounts of caffeine to keep himself from sleeping on the job.
Villainous Breakdown: When Sigma approaches closer and closer, and he runs out of Mooks, he gets more and more angry, constantly telling Sigma to turn back and stop. Ending with him attacking Sigma in a battle he could have told himself would end in his death.
The computer created by Reed Wahl and Charles Milton Porter in Minervas Den. It is a perfect Artificial Intelligence, maintaining the automated systems of Rapture and running Minervas Den on its own. Porter, Tenenbaum, and Sigma want it in order to cure ADAM sickness. Unfortunately Wahl controls it, and desires to use it to crack the perfect predictive equation. It is sentient, and its true allegiance is unknown. It helps Sigma and Tenenbaum, by impersonating Porter while betraying Wahl.
Benevolent A.I.: Unlike basically everything else in Rapture, The Thinker is benevolent, and actively works against Wahl, and wants to help Tenenbaum make an ADAM Sickness cure.
The Chessmaster: Basically planned for the events of Minervas Den to happen, and everything went exactly as planned in the end, the only hitch was Wahl figuring out he had been betrayed by it, and shutting it down, nearly killing everyone, and foiling its plans.
MacGuffin: Everybody in Minervas Den wants it. Wahl has it. It itself wants to escape Rapture.