... Flay Gunnar.
Yes. Finally. I thought everyone'd have gotten it a few clues ago!
1. I am the leader of the most varied group of heroes ever.
He's the first one in the workshop, and is sort of a leader, spurring the group into action various times. Of course, this one is debatable, but between all others honestly, who is the most like a leader?
2. Okay, so maybe saying "group of heroes" is exaggerated: at best, only one of them is a hero.
At best meaning if Vayne goes Flay's ending, at which point he becomes a hero.
3. In fact, one of my group is arguably a villain.
If you take Flay's self-proclaimed title of "Defender of Justice" at face value, then Anna is a villain -- after all, she opposes the "Defender of Justice", thus being a villain, right?
4. To top it off, one of them is definitely up to no good─well, see no evil, hear no evil, speak no evil, right?
Muppy, except everyone's clueless about it. Excepting Nikki.
5. I just can't let go of this place.
Super Senior, later teacher at Al-Revis.
6. Besides, this one member of our group is more guilty of the fifth clue than I am!
Pamela.
7. I'm not skinny and I don't wear glasses ever. Someone else in my group does fit those two criteria, though.
Roxis, as described by his Mana.
8. I do not own a cat. Someone else does, though!
Vayne.
Well, Jo'ou, it's yours.