While some of the characters can be acquired by other methods, these ones can only be selected after accomplishing certain targets on the "Wi-fi Taisen" mode. Perform the following tasks in order to unlock the stated character.

Effect - Code

Haku - 10 Wifi winning cards.
Jiraiya - Play Wifi 10 times.(Not counting win or lose)
Juin - Sasuke 30 Wifi winning cards.
Kyuubi Naruto - 100 Wifi points.
Sasuke - Win at least one match online, then exit Wi-fi and talk to the guy exactly to the left of Anko. You'll get an item that, when used, unlocks him.
Tsunade - Play Wifi 20 times.(Not counting win or lose)