It sounds like your feelings are going everywhere right now, and you're not sure how to feel. This is a subjective state of mind for you. When you're in a subjective frame of mind, you can't make reliable judgments about new people and new situations. You need to wait until your feelings level out a little, reach a state of calm, and allow you to have an objective frame of mind. Then you can analyze this new person and say, "Is she trustworthy?"
As it stands, we really don't know much about her and so it might be perfectly safe for Kuroi to date her (as long as he doesn't go too fast or let NRE overwhelm him). For the moment, I would just let things play out and see what happens. Sure, keep an eye on her for anything that might be amiss -- and keep an eye on Kuroi. Don't let him forget himself in the flush of new love. Remind him that you're still here, and you still need his love, passion, and reassurance too.
I'm sure you're a little disappointed that it's Kuroi and not you who has met a new love interest right away. Don't despair. These things are so random sometimes, there's no telling who's going to meet a new person first. Your time will come, and you'll be just as nervous and excited as Kuroi. You just need to be patient and be yourself. People will fall in love with you for who you are.
Love means never having to say, "Put down that meat cleaver!"