everyone gets so upset at the idea of kenny calling butters leo it makes me laugh. personally, i find it really cute. i think the leo nickname would only come about once they’re actually veryyy very close? I don't think kenny even knows his real name until butters brings it up suddenly when they’re dating so it makes it special because only kenny calls him that (i think kenny uses this nickname more in private). kenny will still call him butters since its like, a nickname that’s stuck with him.