In order to unlock Saiki you need to perform 5 "Missions" on each enemy until the 4th team
First of all put the game on "Very Easy", it will make the things lot easier
Second, select your team (whatever characters you want) and start playin
Third, the game will ask you for missions to accomplish, por example "Do 3 Rolls Back/Forward" or "Do 3 Jumps"
You then need to do 5 missions on each enemy character, after it you can kill him (each character, not each team, so you need to beat a Team, 5 missions accomplished on each character of it, a total of 15 per team at least)
I am not sure if the game resets the mission count when you lose a character, for example, if you perform 3 missions on Chin while using Iori, and then Iori dies... the mission count over Chin becomes 0 again, and does not remain on 3 (need to test this)
Anyways it is quite easy, if you reduce the level of IA difficulty, just make 5 missions on every single enemy (and then kill them), and Saiki will arrive at you after the third team...