I have Minaya in relentless set. The last time I used her, probably more than a week or two, her a1 is working with relentless. She gives multiple continuous buffs. 

That's interesting. I'd have to check with Minaya again, as she's in different gear now.

I do have Riho in relentless at the moment, and it doesn't work. At least not for me. Which begs the question, why does it work for one, but not the other?

The only reason I can think of for why the skill wouldn't trigger is if she is doing a weak hit.

I'd forgotten about Reinbeast. It works for him as well. Which makes it even more annoying that it doesn't work for Riho

She's void. Can't be a weak hit

I was thinking of Minaya.