Serph's pretty familiar with needing to feed off monsters (though his is more of the, eat them, not drink their blood type) but he would be fine with offering blood too. Since Serph heals from wounds by eating monsters, he's already been snacked on by other Drifters. What's one more to add to the lot? XD

He's also very familiar with having a monstrous other form that was forced on him, so I feel like there's some points of commonality that they can sympathise with :>b

(just note the cannibalism cw with this guy, I have an opt out)