There's a way in which Kotetsu looks at him, reaches for him that makes Barnaby think it'll all be okay.
He's not wholly negative – he's merely a realist. Barnaby doesn't see the whole good in things, in people like Kotetsu does. That doesn't mean he hates all other people, he's merely wary, and with that in mind, that's why it took so long to accept that Kotetsu wanting to help him was a wholly good thing.
There's something in the play of his fingers over Barnaby's palm when it is grasped, callouses speaking of countless lives saved (and buildings broken). There's something in the way he brushes his thumbs over Barnaby's jaw, over his cheeks, over his lips – the way that he guides him closer, kisses him, warms him from the inside out.
Barnaby finds himself lurking in the darkness of his thoughts from time to time – thinking of how he's hurt Kotetsu, how he's made things so very, very difficult – but Kotetsu drags him out with something as simple as that kiss, warm and open and his, because yes, Kotetsu is his.
So he holds his breath and waits for that – waits, for the first time in his life, rather than actively searching, because he doesn't have to for once. Kotetsu will always reach for him, even when Barnaby doesn't realize he needs him to.