Chapter Summary: Kenny can't die, but that doesn't mean he has to stay.
He doesn't take anything with him. He just gets up and goes one day.
Nobody notices at first. Kenny's presence is as common and unannounced as Kenny's absence; people rarely note his presence, as he is often what some might call a 'background friend', and nobody ever notices him go, partly because it happens so often, mostly because everyone knows that he'll return, eventually.
Stan notices though. Because Stan has an itch that Wendy won't let him scratch, and for once, Kenny doesn't answer when he calls. At first he waits a few days, keeps calling, but Kenny remains absent. Then he starts to get mad, the itch bringing out something nasty in him, and he calls Kenny foul names, and ignores his friends when they ask him why he's in such a foul mood. Eventually, though, the itch subsides, and Stan gets used to not appeasing it, and that's when he starts to get genuinely worried about where Kenny might be.
He brings it up with his friends, who wave him off, Kyle still angry, Cartman unconcerned. Except, Cartman is not unconcerned. Like Stan, he too has a secret desire that he feels only Kenny can satisfy, and it would be very, very bad if Kenny somehow knew of this. He goes to Butters, his unwilling confidant, who swears up and down that he hasn't spoken to Kenny in months, let alone on the subject of Cartman's obsession. Though Cartman believes him, this does not ease his growing paranoia, and he waits, fretfully, for Kenny to appear, and make a retaliatory attack against him. After awhile, he too brings up Kenny's absence with his friends, and Stan nods back at him, concern in his eyes, but Kyle is angry, wrathfully angry, and storms off.
But what they do not know is that Kyle's strong emotion on the subject comes from his own regret, rather then still enraged by Kenny's actions. Kyle knows that he flew off the handle, knows that too much of his mother came out of him that day, and that knowledge shames and terrifies him. He fears Kenny's absence is a result of his own cruelty, a fear that he can't bring himself to voice aloud to his friends yet, though he often stops by Kenny's house to see if the teen has returned yet, hoping to make some sort of amends. Time has gone by, and Kenny has still not returned, and eventually, Kyle himself brings this up amongst his friends, and this time a discussion occurs.
But the trail has long gone cold, and none of them know of any place Kenny might go to, and so they sit around the cafeteria table in silence, stewing in the fact that, no matter what their desires, no matter what their intentions, nothing can be done about it, because none of them have any idea how to find Kenny and bring him back to them. And so eventually they stop talking about it, and nobody brings it up at all.
Too much time has gone by, and Kenny is gone.