Aslan the Lion is a character from The Chronicles of Narnia. In South Park, he appears as the leader of the Council of Nine in the "Imaginationland" trilogy, as well as the leader of the lions at the zoo in "Here Comes the Neighborhood". In "Imaginationland", he scares Butters Stotch into using his "Key" powers to revive Santa Claus, and later other good characters, so the good creatures could defeat the evil characters.
In "Here Comes the Neighborhood", he seems to have a huge sense of humor, even laughing at every joke he hears. In "Imaginationland", his sense of humor is largely gone, replaced by a more serious, commanding attitude.