Hmm, Kang...he's probably the closest Marco comes to trusting someone outside of the Animorphs. Of course, since Marco's still a paranoid bastard that's...not necessarily a big amount of trust. He trusts Kang to not get him killed needlessly, but he doesn't really trust him with personal stuff. for instance, he's not really going to mention the whole possibly-failing-his-eval thing. He's also noticed that Kang can be a pretty sneaky guy, so he's aware that Kang could have his own agendas going on.

He doesn't really know what to think about the fact that Kang's become friends with his mother. That's just kinda weird.

Also, Kang's kinda more like a boss than a friend. He doesn't really go and 'hang out' with Kang or anything.