Reading through all the Richie and Eddie stuff, there's this recurring beat where Richie's loudmouth bravado directly covers for Eddie's anxieties, and Eddie's hypervigilance somehow catches the things Richie's bluster would miss. It's less about grand declarations and more about how they operate as a unit, a weirdly efficient one. The Losers' Club as a whole has that bond, but these two show it through action and reaction, not just feeling.
I keep thinking about that scene in the clubhouse after the Neibolt house, in the book. Eddie's arm is broken, he's terrified, but he's still snapping at Richie for being an idiot. And Richie, instead of snapping back with a mean impression, just... stays there. He makes a joke, but it's softer. It's loyalty as a default setting. They don't have to think about choosing each other's side; they just are on it. The horror of It tests that by literally trying to pull them apart with their deepest fears, and their friendship is the thing that keeps them anchored, not to Derry, but to each other.
What gets me is the adult timeline. They all forget, but when they remember, that dynamic slots right back into place like no time passed. For Eddie, Richie is a direct line back to a self that was braver, or at least had someone to be brave for. For Richie, Eddie is the reality check he never listens to but absolutely needs. Their loyalty survived memory wipe and trauma, which is maybe the ultimate fictional test of friendship.