For the first set of characters to be available, you have to beat Story Mode. To get the second set of characters, you need to get first place in Time Attack and unlock all the characters from the first set. To get the third set you need to place first in Survival and, unlock all the characters from the second set. To get the final two characters, you must unlock all the previous characters and defeat the extra ten Story matches. They can all be bought in Anko's shop.

Akamaru 2,500
Gai 3,500
Haku 3,000
Iruka 2,000
Kankurou 2,500
Karasu 2,500
Kyuubi Naruto 5,500
Mizuki 15,000
Neji Hyuuga 2,000
Orochimaru 50,000
Sharingan Kakashi 7,500
Sharingan Sasuke 100,000
Zabuza Momochi 4,000