There are many, many playable characters in Naruto Shippuden: Ultimate Ninja Storm 2, some more hidden / secret than others. In this video you'll learn how to unlock two of the more difficult characters to unlock: Lars Alexandersson and Minato Yamikaze.
From the Creator:
This is a quick video on how to unlock the 2 last characters Lars and Minato. I couldn't get me unlocking Minato on camera since I didn't record that fight, But here's how to get them:
600000 SP - Lars Alexandersson
580000 SP - Minato Namikaze
560000 SP - Sasuke Uchiha Akatsuki
540000 SP - Killerbee