"He's helpful like that. Friendly, much of the time." Especially since Haibara is closer to his age than Nanami is. Nanami is also, as he's overheard Ichiji put it, the adultiest of adults. That vibe means he gets treated a different way. He's also gone on missions with Inumaki.

"He's taken to greeting new jujutsu sorcerers. It's because of your lack of cursed energy and that you found me that he didn't meet you first."