Take the following steps to unlock each of the 18 secret characters in Naruto Ultimate Ninja Storm.
1. Gaara: Complete “Burning Will Of Fire” in Flashback Mission: Leaf Destruction Arc.
2. Itachi Uchiha: Complete “Fatal Encounter” in Flashback Mission: Tsunade Search Arc.
3. Jiraiya: Complete “A Match With Pervy Sage”.
4. Kabuto Yakushi: Complete “A Whirling Rasengan Explodes” in Flashback Mission: Tsunade Search Arc.
5. Kakashi Hatake: Complete the Genin Training Arc.
6. Kankuro: Complete “The Aburame Clan” in Flashback Mission: Leaf Destruction Arc.
7. Kimimaro: Complete “Battle With The Sound Begins” in Flashback Mission: Sasuke Chase Arc.
8. Kimimaro (Curse Marked Second State): Complete “A Duel That Risks Their Bond” in Flashback Mission: Sasuke Chase Arc.
9. Kisame Hoshigaki: Complete “Toad Sage Shows His Ability” in Flashback Mission: Tsunade Search Arc.
10. Might Guy: Complete “Odd Beast Battles a Scourge” in Flashback Mission: Tsunade Search Arc.
11. Might Guy (Eight Gates): Complete “A Duel That Risks Their Bond” in Flashback Mission: Sasuke Chase Arc.
12. Naruto Uzumaki (Nine-Tailed Fox): Complete “A Duel That Risks Their Bond” in Flashback Mission: Sasuke Chase Arc.
13. Orochimaru: Complete “Sannin Rock Heaven And Earth” in Flashback Mission: Tsunade Search Arc.
14. Rock Lee (Eight Gates): Complete “A Duel That Risks Their Bond” in Flashback Mission: Sasuke Chase Arc.
15. Sasuke Uchiha (Curse Marked Second State): Complete “A Duel That Risks Their Bond” in Flashback Mission: Sasuke Chase Arc.
16. Temari: Complete “The Chunin Exam Sleeper Pick” in Flashback Mission: Leaf Destruction Arc.
17. The Third Hokage: Complete “Unprecedented Mega Battle” in Flashback Mission: Leaf Destruction Arc.
18. Tsunade: Complete “Tsunade Finally Opens Her Eyes” in Flashback Mission: Tsunade Search Arc.
Thanks again to Narutovshaku for sending in the list!