Kosso works best in small groups, particularly groups of two, and excels specifically at protecting other people while they do more complicated things (turns out bodyguard training comes in handy). That said, he can pretty much go anywhere: escorting one of the techies, exchanging Kesik, or helping out in a firefight. There's a decent chance he'll probably just stick with Tanya anyway, but it's all up in the air as far as I'm concerned.