SPOILER ALERT! He is Takehaya Susano'o - true form of "Yuki Terumi". Sankishin Susano'o is his original body, when "aparted" Yuki Terumi - manifestation of it's "will"...
Haha, I never thought about using 3C as corner juggles! A lot of the combos feel more impractical over all for that little extra damage, but those two seal-unlock combos are fantastic.
they're seals, and they represent his special moves basically, whenever his normals connect(even if they're blocked), a cursor highlights one of those seals and goes forward as you connect more normals. connecting a Drive normal unlocks whatever seal the cursor is currently highlighting at the time(if it's already unlocked, it powers up the special move, although it doesn't happen for every seal), and then the cursor resets. he also has a distortion that unlocks every special move, but you can only use it during his overdrive. also, you have to unlock his special moves every round.
From what I understand the seals above his heat gauge are his special moves. His drive attack unlocks these seals based on when you do your drive during a combo. The seal that is highlighted changes with every successful normal command you land regardless if it's blocked or not. It should be noted that the special moves can still be used if locked, they are just weaker and lack certain special properties they have unlocked or leveled up. Each sealed ability can be leveled up 3 times which adds extra damage, more hits, etc. The number of seals you have unlocked also affects one of his distortion drives as I'm sure you saw. He has a distortion drive he can only do in OD that unlocks all of his seals as well. Very unique and interesting gimmick to say the least.
No because his kit is too complex to work in Cross tag... Legit wouldn't be a problem if the switch button wasn't the fucking drive button from blazblue. But lets say they made his kit simple and got rid of his unique locks. Then he'd be a boring shell of who he is in central fiction. And honestly the main reason I love him other than his feral straight up savage fighting style and attitude is his unique locks that you have to weave into your neutral combos to unlock drive moves. Without that aspect and with cross tag being simple combos I would hate it if they put him in the game. I honestly think it would tarnish the image I have of this guy if that were to happen. His home is in Central Fiction. And maybe in some future blazblue game that isn't cross tag.
All characters in Blazblue are tournament legal. They never break the characters. Infact characters like Susano'o tend to come out underpowered compared to the other characters despite being a boss (except Hazama, he came out Overpowered).