I tried to look up what a type 4 anti-hero is, on tvtropes, but unfortunately the database was down.
So it's hard to say anything about that, simply because the term covers a great variety of characters.
I wouldn't really consider Tomoe a leader. She herself doesn't think she is one and certainly doesn't want to be one. It's the other characters that see her as the leader, even though she hasn't taken the lead yet (I think). She could certainly become one with the proper resolve, though.
About Shin being her foil, I don't know. Let's not forget that he hasn't been a gantzer for more than a day. I got the impression that he wanted to show off infront of Keiko. Given that he told the first-timers that he knows more about what's gonna happen than they do, despite the fact that he doesn't even remember what happened in the last hunt (and as i said, only started doing this 20 hours earlier than the newbies). He did try to lead, but look at what became of his plan. He abandoned it as soon as Keiko wanted to save civilians. I think it's too soon for him to lead. I mean he doesn't come of as the kind of guy who knows how to lead yet.
I wouldn't say the readers were turned against Shin because of what happened. To me, he comes of more as a kid that got bullied a lot in the past. And now, after having gotten this new power thanks to the suit, he's somewhat becoming a bully himself. He used the suit to kill his father immediately after the first hunt, and he jumped to violence as soon as he got head to head with Takezo and Ryu. Something you'd expect of a child that got bullied a lot. Gets into martial arts with the purpose of learning how to defend himself, only to end up attacking people instead.
This and his attempts to show off in front of Keiko let him appear younger than he really is. I'd expect such behavior from a teenager, but maybe I'm just expecting a more grown-up attitude from a 21 year old.
Which might also be the reason why I wouldn't consider him cute. Then again, I'm a guy.
In any case, I don't feel like he's on the road to becoming a villian. He might end up there at one point, but I think it's too early to say that this is where he's headed. We're not far enough down the road to say that this is where it leads. Right now he just seems overwhelmed with the power he has now. Which I personally like, since no character has really done that so far, even though it's a reaction you'd expect.
I noticed his insecurities about his height when he lashed out at the team for thinking that he's weak because he's the shortest one there. Another interesting trait. But what about Keiko? Wouldn't it eat at him to some degree that she's taller than him?
PS: I haven't forgotten about the update. My patience for waiting ran out two days ago, but I didn't have the time to write something. But you can expect it tomorrow...or today, depending on where you live (Thrusday evening).
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"