Although they have their differences with each other, and rip on each other plenty, whenever things get really tough - or really, every Wednesday in South Park - Stan Marsh and Kyle are pretty much always there for each other. They've been best friends since they were kids and hang out together all the time - and so do their parents as well. Whether they're saving the world together, or just saving each other's lives from whatever's been conjured up into town this week, these guys always seem to have each other's backs.
It's not all rainbows and sunshine for Stan and Kyle though -- from time to time, Stan's ego can get in the way of their friendship and Kyle can only deal with so much of Stan's negativity. Still, as long as they aren't asked to name a prehistoric ice creature, these two can usually keep it together and put their super best friendship above everything else.
The rivalry between Eric Cartman and Kyle has been a recurring theme from the beginning, but has become significantly stronger since Season Four. Cartman appears to have a very strong hatred for Kyle, and shows that he has no tolerance for Kyle's Jewish faith. Kyle hates him in return, but neither one of them seems to realize the effects of their almost poisonous relationship, and continue to live in close contact with each other.
In Season Five, Cartman and Kyle's relationship meets a turning point, going from a relationship of them simply ripping on each other constantly to near hatred simply because Cartman caused Kenny's semi-permanent death. It was around Season Eleven that they began their descent from like-hate to full on hatred. However, in the "The Death of Eric Cartman", during the scene where Cartman has Butters apologize to all of his peers in his stead (as he believes he is a ghost), he seems to wish to recant some of his earlier misdeeds towards Kyle (and obtain his forgiveness), and seems genuinely moved upon seeing him.
Kyle is often put in the shoes of the protagonist to Eric Cartman's antagonist. For example, in "Cartoon Wars Part I", the two of them work against each other; when Cartman tries to get Family Guy canceled, Kyle tries to stop him. The two of them often have arguments about morals, like in "Up the Down Steroid", where Cartman decides to pretend to be mentally handicapped so he can win the "Special Olympics".
Mr. Hankey made his first appearance in "Mr. Hankey, the Christmas Poo" and was seen as generally unknown to the other characters besides Kyle Broflovski and Chef, but after the episode's events he was popularized and by "Merry Christmas Charlie Manson!" there are several TV specials about him and he even takes a more Santa-esque role at malls, but he makes no actual physical appearance in the episode.
In "Chef's Chocolate Salty Balls" the boys meet him in the sewer and he explains how the Hollywood people, led by Robert Redford were destroying the ecosystem. Kyle's and his calls are ignored and turned into a movie starring Tom Hanks called "Mr. Hankey and Me", and Mr. Hankey eventually dies. Using Chef's Chocolate Salty Balls to revive him, he uses magic to get rid of Redford and the Hollywood people, by transforming into a wizard (in the style of Mickey Mouse from Fantasia) and raises all the feces from the sewers to attack the Hollywood folk with swarms of fecal matter. While this successfully got rid of the Hollywood folk destroying the town, it ironically and very disgustingly covered the whole town in feces. Kyle here refers to Mr. Hankey as his second best friend.
Mr. Hankey hosts "Mr. Hankey's Christmas Classics", which features a new rendition of his theme song. He returns for "A Very Crappy Christmas" in which we meet his a wife Autumn, who has a drinking problem. We are also introduced to their three kids: Cornwallis, Amber, and Simon (who was born with a peanut in his head) Mr. Hankey later sings "Circle of Poo" with Corwallis and helps the boys make their Christmas short. He reappears briefly in "Red Sleigh Down" with his poo-choo train. This was his final appearance in the series until "201" when he is revealed to have orchestrated the previous episode's plot to hide Santa Claus in a Bear costume instead of Muhammad.
He also appeared in "I Should Have Never Gone Ziplining", where he was seen rescuing the boys from extreme boredom on the boat they were on. He took them away in his magical "Helicrapter". After traveling so far, Hankey took the boys further on his "Seven Turdy Seven". From there, Hankey took the boys home on an hour-long ride on the Poo-Choo Express.
He reappears in "The Problem with a Poo" where he is the director of the Christmas pageant. He starts sending angry, crude tweets after his budget is cut and is later fired. After an unsuccessful hearing and Christmas concert, he is banished from the town of South Park, and ends up in the town of Springfield from The Simpsons, where his actions would allegedly be tolerated.
Kyle's always been popular with the ladies in South Park - early on, the popular Bebe Stevens thought he had a nice ass and he later returned the favor by admiring her new breasts, but their relationship ended quickly after a forced first kiss. She wasn't afraid to make him a casualty of the Pleases and Sparkles Club committee when they made the List of Cutest Boys, however.
Past Love Interests
His first real relationship was with the home-schooled girl, Rebecca Cotswolds, who he introduced to the art of kissing... but it didn't work out so well, and she ended up kissing lots of guys. No details on the breakup are known.
He's been a swingin' single ever since, but he did go on a date with Nichole Daniels and displayed attractions to Ms. Ellen and Lisa Berger with the other boys. He seemed on the cusp of a relationship with Leslie Meyers, until her secrets were revealed to the town.
He developed feelings for Heidi Turner when he tried to help her escape her abusive relationship with Cartman, finding they had a lot in common, but became bitter when she instead rejected him for his Jewish heritage and reunited with him instead. He cited his feelings for her as a motivation toward his dislike of Canadians.
Despite his charming personality, Kyle has never made it passed one date with any of his love interests on-screen, meaning he's yet to have had an official girlfriend.
Other than his two closest friends, Kyle's closest friend has always been the local poor kid, Kenny McCormick. When he's not hanging out with them, he's usually sticking up for the schoolyard's punching bag, or hanging out with a certain handi-capable comedian.
He also had a very strong friendship with his late, great school cafeteria chef.
Kyle has an extended group of friends that he hangs out with at school. He regularly catches up with them outside of school though, usually to see what crazy new thing has come to town, or to simply watch the newest Terrance & Phillip episode. His friendships with these characters haven't been explored in focus, however.
Kyle has a unique set of minor friends, who he shares a mutual liking to. Whether it be a eligious deity or a Zypod from the moon, they're always there to cheer Kyle up.
He also befriended a Prehistoric Ice Man from the year 1996, who he eventually had to part with. Whether they remain in contact is unknown.
Kyle's relationships with these characters have never been (or currently isn't) quite clear, and his mutual feelings about them is usually up for debate amongst fans.
Kyle has no recurring enemies besides Cartman, however, he has had a number of short-lived rivalries