I've written a few 'Banana Fish' stories myself, and the thing I keep coming back to is balancing their dependency. Ash isn't just some guy who needs saving—he's a survivor, hardened and sharp. Eiji isn't a naive child; he's compassionate but grounded. The best fics I've read don't infantilize Eiji or make Ash too soft. They argue, they misunderstand each other's trauma, and the connection grows from navigating those gaps, not erasing them.
A specific trick? Give them a shared, quiet activity that's not about their past. Maybe Ash teaches Eiji how to play chess, but Eiji keeps breaking the 'rules' with unexpected, kind moves that throw Ash off. Or they cook together, something simple, where the focus is on the task, not a heavy conversation. The chemistry sparks in those unguarded moments when their walls are down because they're focused on something else. It feels more earned than just having them confess feelings under a sunset, you know?
One more thing: dialogue. Ash is terse, guarded. Eiji's English isn't perfect, so his phrasing can be simpler, more direct. When you write their conversations, that contrast can do a lot of work. Eiji's straightforwardness cuts through Ash's defenses in a way a more eloquent character's speech never could.