Just found this story now; I'm loving it so far (well, okay, I'd found it before, but this fic ended up in my "Huh, looks like an interesting story, and I've liked previous stories that this author has written, but will inexplicably put this into my list of stories to read later" pile. Which is a shame.)
It is kind of interesting to see the dynamics of a Grail War in which the Grail is (probably) uncorrupted; I got into F/SN through fanfiction, so I had that little plot twist spoiled for me right off the bat, so I never really had a chance to try and figure out who I wanted to root for, since I knew that any of them winning would probably be a bad thing.
This Kotomime, unless I missed something, seems to be pre-Zero in terms of characterization (unless I missed something, though he did say rejoice, and that's never a good sign).
Which is almost a shame, since for however much of a bastard he is, he's kind of fun once he embraces his nature, and seeing him interact with the HP cast (as well as your OCs) might be interesting.
His character's still interesting per-revelation, though, so, uh, don't take that as a complaint or anything.
I... think I'll wait to comment more until I'm sure that my comments won't retread ground covered by the SB thread.
Though, one thing I put in my list of things to say; Dumbledore thinking that he had intended to "unleash a team of bros unlike the world had never seen?"
That statement... I liked it, but it almost felt... jarring?
I mean, I can kind of imagine him saying it, because being a bit odd is entirely in character for him, but it took some thinking for the thought to fit.
(I feel vaguely like I'm rambling, but I just wanted to provide something a bit more substantive than a chain of likes)