When you meet all the requirements, you will face an extra boss after defeating Seth. Like Seth, these will be powered up versions of the normal playable character. All number requirements must be met before fighting Seth. All numbers assume the normal 3 round setting.

Akuma: Never lose a round, and get 1 Perfect victory.
Evil Ryu: Never lose a round, get 1 Perfect victory, and defeat Seth with a Super or Ultra combo.
Gouken: Never lose a round, get 1 Perfect victory, get 10 First Attacks, and have 5 Super or Ultra combo finishes.
Oni: Never lose a round, get 1 Perfect victory, get 10 First Attacks, have 5 Super or Ultra combo finishes, and defeat Seth with a Super or Ultra combo.