Assuming that by "which boss" they mean someone who was actually a boss or mid-boss in the games, and that I didn't pass them over... the missing person...
...probably has very good reason to be jealous of everyone who made it into the picture.
Heh, I tried finding them all before reading the comments (except for the Lunatic, I knew that would just be madness,) and figured Nitori would be using optical camouflage (why else would Nitori be hard?), but I was looking for Nitori as a dotted outline, or as someone holding up a poster of the background or something, not just being at 20% opacity. I didn't expect her to just be a slightly darker blue shadow on a different character.