The following quote comes from this strip, featuring Jirix and the Anti-Villain Redcloak talking about their Card-Carrying Villain partner Xykon. Coincidentally, this strip occurs just after several in which the paladin O-Chul and Redcloak had served as different but excellent foils for each other.
Jirix: But... aren't we all on the same side? Redcloak: That is a complicated question... Our alliance with Xykon is one of the most powerful tools we have, and we cannot afford to screw that up. That does NOT mean we should trust him. I know he seems funny and charming, but believe me, when you see for yourself the depths to which he'll sink, you will never sleep well again.
Elan and Nale; polar opposites in Character Alignmentinvoked, both have a flair for the dramatic, similar class abilities (though Nale's Complexity Addiction means he took a more roundabout way of getting them), similar relationships with their respective love interests (who are also foils), and a tendency toward being dumb (though Nale will vehemently deny he is anything but a genius).
Elan and his father Tarquin, who are complete opposites when it comes to alignment and intelligence.
Roy and Thog. Both are melee fighters carrying heavy weapons and both like to talk, but while Thog is a Dumb Muscle and a munchkin-build, Roy is a balanced character: he is pretty smart for a fighter and is a charismatic leader.
It's worth mentioning that Nale deliberately themes his roster around being "Evil Opposites" to his brother's companions, while Roy refuses to accept that Thog is one to him, because that would suggest they're similar in all but a few key ways.
Com'c: Block and Krixwell are contrasts to each others. For example, while Block has yet to even smile in the comic (although he does have a slight smile or three in the December 2013 calendar), Krixwell has yet to say something without smiling.note He does occasionally get the "neutral closed mouth", though.
The main characters, Annie and Kat. Magic Versus Science is a recurring theme of the story and setting, and is represented in the two of them. Annie, who converses with psychopomps and has connections to the ether and the forest, represents the supernatural. Kat, a Gadgeteer Genius who grew up in the Court, represents technology. Personality wise, Kat is more outgoing and interested in pop culture and romance, a stark contrast to Annie, especially at the beginning of the story when she was more stoic. Annie and Kat have both had problems fitting in — Annie because of her personality, lack of social skills, and inability to relate to most people her age, and Kat because of resentment over her grades and suspicion that, as the daughter of two Court teachers, she was being favored. Annie and Kat balancing each other is a recurring theme in the Treatise pictures, where they are also associated with the sun and moon, respectively.
Zimmy and Gamma, "two strange girls", work as a pair of foils to Annie and Kat, the other "strange girls". Jack is Kat's own foil: both are technical-minded, curious and slightly mischievous people who have a dramatic meeting with the supernatural world. Kat is baffled but eager to understand it, while Jackbecomes suspicious and frustrated after a traumatic experience with psychic powers.
James Eglamore's close female friends Surma Stibnite and (miss) Jones: Surma was a literal Fiery Red Head while Jones has compared herself to a stone because she claims to feel has no emotions she's also existed since the earth was a ball of lava and cannot be harmed much less killed while Surma died young. Hilariously James cannot fathom why his ex Surma liked, let alone married Anthony Carver while he's perfectly happy being friends with Jones.
Reynardine the fox and Ysengrin the wolf: Rey is trapped in a plush wolf, likes humans, and has become friends with his "master" Annie; Ysengrin willingly asked his master Coyote, who is constantly insulting him and eating his memories for a humanoid exo-skeleton so he can be stronger (ironically Ysengrin hates most humans and his dependance on the armor appears to be atrophying his real body).
Reynardine and Hetty, who despises her "master" and wants to kill him to be free Hetty's "master" is a young boy who has no idea Hetty exists and it's implied Hetty killed his sister, her original master. Rey's real form is a land-dwelling mammal (fox) while Hetty's appears to be a water-dwelling crustacean (shrimp).
In The Indefensible Positions we have Foil (no pun intended) and Frank. It's revealed that Foil was Frank's imaginary friend. After they split up, without him Frank's personality was changed to completely opposite of Foil's.
Lightbringer, a superhero who believes in an objective morality and bent on doing the right thing; and Darkbringer, a supervillain who believes in an objective morality and finds Lightbringer to have misplaced what is right and wrong.
In The Dreamer, there are several foils: Alexander and Alan, Alan, and Nathan, Beatrice, and Alan, Yvette and Liz, Beatrice and Yvette, Beatrice and Liz, and Ben and Alan.
In Girl Genius, Gil and Tarvek are increasingly making foils of themselves. Tarvek is a well-bred, thoughtful, mannerly aristocrat; Gil is an energetic, sometimes rash, son of a self-risen dictator. They both are in love with Agatha, they're both powerful sparks, and they both want to "ally with" the last Heterodyne and bring peace to Europa — but in different ways. They were also friends at school and now they apparently hate each other.
Squid Row: Randie has an organized goal-setting friend who tries to persuade her out of her lack of planning.
In Bobwhite, Ivy goes to the library at Redding University (rival to Bobwhite University) and gets annoyed by three guys. Marlene and Cleo realized that these guys are the evil reverse-gender doppelgangers of themselves (though Ivy interjects that her supposed counterpart really looks nothing like her). Which makes things kind of weird when Ivy later starts dating one of these guys (the one who's pretty much a male version of Marlene).
Kankri was intended to represent the worst of social justice bloggers (being overly obsessed with details, giving sermons instead of having conversations, being generally arrogant and consescending) and Porrim the best (articulating her point clearly and concisely, sorting through bullshit and real problems, and generally having a life outside social justice).
John and Rose have foils in Tavros and Vriska, who could be described as "John and Rose if they grew up on a dystopian Death World." John and Tavros are both Breath heroes with similarly sweet, naive, trusting, and amiable personalities; Rose and Vriska are both Light heroes who exhibit questionable morality and get played by Doc Scratch even while they're trying to play him.
Jade has Kanaya (both Space players, both had highly unusual guardians, woke up early on Prospit, and have similar theme colors).
Dave and Karkat serve as foils to each other, both being mouthy smart-alecs (in totally differentways) who use snark to cover for their raging inferiority complexes. They're both Knights, have red as their color motif, and even have similar taste in girls (Terezi, Jade).
The B2 kids serve as foils to the B1 kids, especially Dirk/Dave and Roxy/Rose, and the A1 dancestors do the same for the A2 trolls, especially Kankri/Karkat, Latula/Terezi, Aranea/Vriska, and Rufioh/Tavros.
Expanded upon a bit with the B2 kids, who later act like the B1 kids post-Character Development. Roxy is initially a Foil to Rose, though later becomes one to Dave, with Dirk foiling Rose. Jane mostly foils against John - their love of pranks, living up to their past generations, and their happy-go-lucky personalities - with Jake foiling Jade, mostly in their upbringing and whimsical, detachedhammy personalities.
Corrick and Vesper are the most straightforward. One's a Really 700 Years Old badass with supernatural powers, a realistic, if not cynical outlook of life and disdain for his job. The other is an Action Survivor young girl who's slowly growing into a Blood Knight.
Corrick and Azeel: while both are Aurans changed by magic, the former dislikes his gift and aims to protect, while the latter enjoys himself while sowing destruction.
Manly Guys Doing Manly Things: Commander Badass and Tank are foils to each other. Both are burly, blonde and highly masculine super-soldiers, but where the Commander's masculinity is confident and positive, Tank's is insecure and toxic.
Stand Still, Stay Silent: The two non-immune members of the exploration crew, Tuuri and Reynir, are this to each other. They both have a backstory that can be boiled down to Small Town Boredom stemming from the fact that the space between safe settlements is Plague Zombie ridden and only those who are The Immune can move between them freely. Tuuri dealt with it by really wanting to explore non-safe areas and enthusiastically joining an expedition intended to do just that. Reynir, meanwhile, would have been satisfied with seeing a country other than his own and sticking to said country's safe areas. He ended up with the exploration crew by complete accident and is much more cautious about getting infected than Tuuri.
Rachel to The Hero, Baam: Baam is determined to do whatever it takes to do what's right even against impossible odds and succeeds due to hidden powers/luck/destiny, whereas Rachel has none of those three and is just painfully ordinary, and is determined to get ahead at any cost, up to and including morally heinous things.
Hoaqin to Baam: Baam keeps sacrificing himself for everyone, whereas Hoaqin sacrifices everyone else for his own advancement.