Kenny is very flashy and romantic. He is close to Stan and Kyle, and secretly (Sometimes exposed) has a crush on Stan, despite the childhood friendship. He is the first blonde major character (Jimmy is the other, but is a antagonist) and it gets alot of romantic attention towards him.
Stan: Kenny and Stan are pretty close, and sometimes in romantic relationships with eachother. They are seen interacting very often, and Kenny is sometimes jealous of Kyle's higher friendship with Stan. Kenny, along with Kyle, help him the most during his loss of consciousness after his fight with Jimmy during Unexpected Change.
Kyle: Kenny and Kyle don't interact much, but when they are, they do get along pretty well. Sometimes, Kenny is jealous of Kyle being the Co Leader, but this is uncommon.
Cartman: When Kenny and Cartman do interact, it isn't positive, and in The Walking Dead, Cartman killed himself, causing Kenny to also get eaten, by ruining the weight balance of a tilting roof.
Lenny: Kenny and Lenny have never directly interacted, and their relationship is unknown. Kenny does help keep zombies away from an injured Lenny in The Walking Dead.