I reread the teammate introduction and dialogue, and didn't see anything suspicious. Well, beyond Kato being full social spec and clearly an excellent liar. Still, if he was an enemy agent, volunteering all the information he did wouldn't have been necessary. In principle, since he doesn't care about the results of this event, he'd be the most amenable to being turned, and his espionage specialization also makes him less likely to reject such an offer out of principle. Still, if he were working for Mist, why offer all this information? He could've easily just not said about half of it and we wouldn't have been any wiser. In particular, identifying himself as the person who screwed Hazou last event provides little benefit if he intends to betray us later anyway. It makes sense if he's worried that Shin could reveal it, but that shouldn't be a problem if they're working together.
On the other hand, they've already worked together, and Shin would absolutely figure out (or outright be informed) that we'd get minor village people in our team, and make an offer to Kato in advance. In that view, his behaviour could be seen as being as honest as he can afford to be - the foundation of any good deception. Not sure how we'd approach testing for this, though. If he was going to betray us, how would he do it?
Or maybe, there won't be any betrayal strictly speaking, but the opposing Blue Team has Downfall in it, and they have a pre-arranged way of tracking Kato? In that case, we really want to get a headstart on detecting their approach while we're still locked in place by the proctor.