You have Stan, who is your best friend, whom you love; you have Eric, who is your enemy, whom you hate; and you have Kenny, who is just someone you keep around so that, when Stan and Eric are busy, you aren't alone.
Kenny is your backup friend, your friend in reserve, and then one weekend when you get tired of sitting on a lawn chair and watching him blow things up in his backyard, your makeshift makeout ken doll.
And when you see him around and see the way he looks at you when you won't look at him you don't feel guilty—because it's just a nothing-thing and he's just a sometimes-something, just the insignificant other—and you won't feel guilty if that means making it/him mean something.