Lenny is silent, strong and very decisive. He can even kill all of his friends with barely any sympathy or regret, showing barely any remorse over decisions he makes. The reason for his lack of speech is never revealed, although he does "speak" when injured, singing or during important situations.
Stan: Lenny doesn't really interact with Stan much despite him being leader 1 of The Dreads, although it is assumed he cares for him. When Lenny dies in Choking Short, Stan mourns him and also tries to save him in The Walking Dead. Stan also is shocked when he witnesses Lenny shoot Clyde, and even fights with him for doing so.
Kyle: Lenny doesn't interact with Kyle much, but does seem to care about him a little bit. He attends Kyle's birthday and gives him a RC car.
Kenny: Lenny and Kenny have not directly interacted despite their close names. Kenny helps fight zombies while Craig and Token try to get him out of a beartrap in The Walking Dead.
Cartman: Lenny pity's Cartman and does not care for him like most of the Dread members.
Craig: Lenny and Craig seem to have a pretty good friendship, and Lenny joins Craig's rebels during Riot, however he kills (possible) rebel member Clyde, and later attempts to kill both Stan and Craig by shooting them, however is killed by a headshot from Craig, who almost immediately fell down and died from blood loss.
Clyde: Clyde and Lenny rarely interact, most prominently in Riot, when Lenny holds Clyde at gunpoint, and Stan attempts to stop Lenny, but fails as Lenny shoots Clyde in the chest and head, which initiates a fight between Stan and Lenny.
The Walking Dead
Scary Campfire Stories
Kyle's Birthday Short
Lunch Fight Short