pic :  http://i259.photobucket.com/albums/hh306/tuanlesoleil/20080713082211.jpg

before fighting Seth, you will be fighting sub boss, your rival - after a certain amount of matches (sorry, didn't keep count), the CPU will chime in with "Now Fight Your Rival!" and you'll face off against a character specific sub-boss. The music even gets more dramatic. Although, I was fighting against Chun-Li in one instance and I noticed a remix of her classic theme kicking in, which was pretty cool.

***warning, read this Spoiler at your own risk***

Spoiler for Seth's Moves: He's got...a little bit of everything.

-- He's got Dhalsim's C.FP.
-- He can throw Sonic Booms, including two consecutively.
-- He can Shoryuken.
-- He's got Urien's Shoulder Tackle as well as the Dive Knee, but he does it by bouncing off the wall and then flying at you...he can cover quite a bit of distance.
-- He's got the Urien/Gill anti-air elbow, and he can chain this into a mid-air sequence that ends with a Shoryuken. However, half the time he missed the SRK.
-- His throw is an uppercut punch to the gut, which sends the opponent in the air, and then he jumps up and SPD's them to the ground.
-- I saw him whiff what looked like SPD animation twice. Maybe the "regular" throw I mentioned before was the SPD, I'm not sure.
-- He's got a Bison-like teleport.
-- He can suck you towards him and then attack you.
-- He's got the Gill S.MK...? The one where he's got a lot of reach...but if he hits you, then your character freezes and gets hit by multiple invisible hits.
-- His super or ultra...he fires a burst of air at you from his midsection. If it hits, your character gets sucked up into a whirlwind and then sucked into Seth's midsection...and then he spits them out. The air burst can be blocked and jumped over with the right timing, but if he catches you in the air at an inopportune moment...he will hurt you with this.
-- While I don't think this is unique to Seth, I got hit out of ultras/super combos A LOT.
-- The first round he takes it easy, but in round two he turns up the heat big time.
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Spoiler for Rivals:
Guile's rival is Abel. C.Viper's rival is Chun-Li. Ken's rival is Rufus. There's no special pre-fight intro or dialogue or anything like that.

... he spits you out?  as in out of his mouth or out of that mid section..... how do you fit in there?  does your player like shrink or something?

I'm not too keen on this guy's movelist.

Most of his moves are borrowed?

Capcom pulling a Shinnok?