Just to clear up a misconception about the ranking system, the letter is determined by your power, while the + and - is based on your combat prowess compared to other monsters in your rank. If you're particularly effective, you get the +, if your abilities are poor or you have a significant weakness, you get a -.
So, Valravn was C Rank because he had 163 power. He got the + (even in his bird form) because his dark matter slash gives him extreme offense, he has high mobility with his shadow teleportation, and he's immune to 'immaterial' powers (even level 5 Nico couldn't teleport him).
Notably, his rank did not change from bird form to knight form, it was still C+ despite the notable increase in his combat effectiveness from his armour and sword. He did not become B- or C++.
Alice would be B+. She was level 2, which is equivalent to C Rank, and her monsterfication probably boosted her power to B Rank, or maybe even higher. Because of how powerful her trait is, and how good she is with it, she gets the +.
Danu is also definitely A+ with her golem, which is why I suspect Alice may be A+. I'm not sure if any B+ monster would be able to take on an A+ monster. Either way, she's definitely not A-.
P.S. 90% sure this is explained in the H.E.R.O. section of the game, but I don't remember exactly where.
