I recruit El Guapo with boost:

Boost 2: When El Guapo comes into play, characters you control have reinforcement while defending this turn. Return El Guapo to his owner's hand.

So El Guapo comes into play, and his boost text triggers.
In response, I play Die for DarkSeid!, Ko'ing El Guapo.
Now, when El Guapo's boost trigger resolves, will El Guapo, who is now in the KO'd pile, return my hand?

no, as he is not considered the same el guapo when he is in the Ko pile.

agreed

Just found the bit in the Comp. Rules, got a bit dizzy reading that thing before so I thought I'd ask. Thanks.