During combat showdown where I am the defender; I trigger my hidden Here to Help at my controlled battlefield to bring in Graves. Would his defend trigger even when it was past the initial On Attack/On Defend stage?

Yes — Graves will get his defend trigger even though the initial attack/defend step has passed. Defend triggers aren't locked to that first moment; the check is "first time this combat the unit gains the Defender designation" (Rule 383.4.f.2), and per 466.2.c.3.a a unit that becomes present at the battlefield after designations were handed out gains its controller's designation during the Cleanup following the action that put it there.

So the sequence is: reveal Here to Help as a Reaction (hidden cards have Reaction) → it resolves and you play Graves to that battlefield (811.1.d.5 forces him there) → cleanup gives him the Defender designation → his "when I defend, deal damage equal to my Might to all enemy units here" goes on the chain. Being new this combat, it's his first designation that combat, so it fires.

Two things worth remembering: the trigger goes on the chain and can be responded to by the attacker before it resolves, and everyone else present took their defend/attack triggers already because those only check once per combat — Graves only gets his because he wasn't there yet.