There are a few different ways to unlock characters in Soul Calibur IV. For example, the final boss of the game, Algol, can be unlocked by defeating him in story mode.Soul Calibur Wiki: Soul Calibur IV AlgolApprentice is obtained by beating Arcade Mode with either Yoda or Darth Vader.1Up.com: Review: Soul Calibur 4

The majority of characters in Soul Calibur IV must be purchased using gold earned by completing Story Mode. Players begin with 20,000 gold and earn additional gold every time Story Mode is completed on normal or hard difficulty. Each character costs 4,000 gold to purchase.GameFAQs Topic Search: Soul Calibur 4 Unlock (Xbox 360) | PS3

Defeat in Story Mode:

• Algol
• Angol Fear
• Ashlotte
• Kamikirimusi
• Scheherazade
• Shura
• Ashlotte

Purchase for 4,000 Gold:

• Amy
• Cervantes
• Hong Yun-seong
• Rock
• Seong Mi-na
• Setsuka
• Sophitia
• Talim
• Yoshimitsu

Defeat in Arcade Mode with Yoda or Vader:

• ApprenticeGameSpot: Soulcalibur IV Unlocking Characters IGN: SCIV Unlockable Content FAQ - v.2

When you play through the game with a particular character in story mode, you will face a bonus/unlockable character on stage three of five. If victorious, you will unlock that bonus character. Below we have listed out the main characters and their corresponding bonus characters. NOTE: To unlock The Apprentice, you must beat him with either Yoda or Darth Vader in arcade mode.

1. Algol:
   • Level 3 Boss: Angol Fear
   • Main Boss: Siegfried
2. Amy:
   • Level 3 Boss: Kamikirimusi
   • Main Boss: Nightmare
3. Angol Fear:
   • Level 3 Boss: Ashlotte
   • Main Boss: Algol
4. The Apprentice:
   • Level 3 Boss: Angol Fear
   • Main Boss: Algol
5. Ashlotte:
   • Level 3 Boss: Scheherazade
   • Main Boss: Algol
6. Astaroth:
   • Level 3 Boss: Ashlotte
   • Main Boss: Algol
7. Cassandra:
   • Level 3 Boss: Kamikirimusi
   • Main Boss: Algol
8. Cervantes:
   • Level 3 Boss: Shura
   • Main Boss: Algol
9. Darth Vader:
   • Level 3 Boss: Angol Fear
   • Main Boss: Algol
10. Hilde:
    • Level 3 Boss: Scheherazade
    • Main Boss: Nightmare
11. Ivy:
    • Level 3 Boss: Shura
    • Main Boss: Nightmare
12. Kamikirimusi:
    • Level 3 Boss: Shura
    • Main Boss: Algol
13. Kilik:
    • Level 3 Boss: Angol Fear
    • Main Boss: Algol
14. Lizardman:
    • Level 3 Boss: Ashlotte
    • Main Boss: Nightmare
15. Maxi:
    • Level 3 Boss: Ashlotte
    • Main Boss: Nightmare
16. Mitsurugi:
    • Level 3 Boss: Kamikirimusi
    • Main Boss: Algol
17. Nightmare:
    • Level 3 Boss: Scheherazade
    • Main Boss: Siegfried
18. Raphael:
    • Level 3 Boss: Shura
    • Main Boss: Siegfried
19. Rock:
    • Level 3 Boss: Ashlotte
    • Main Boss: Algol
20. Scheherazade:
    • Level 3 Boss: Angol Fear
    • Main Boss: Algol
21. Seong Mi-na:
    • Level 3 Boss: Shura
    • Main Boss: Algol
22. Setsuka:
    • Level 3 Boss: Shura
    • Main Boss: Algol
23. Shura:
    • Level 3 Boss: Kamikirimusi
    • Main Boss: Algol
24. Siegfried:
    • Level 3 Boss: Scheherazade
    • Main Boss: Nightmare
25. Sophitia:
    • Level 3 Boss: Ashlotte
    • Main Boss: Algol
26. Taki:
    • Level 3 Boss: Shura
    • Main Boss: Siegfried
27. Talim:
    • Level 3 Boss: Ashlotte
    • Main Boss: Algol
28. Tira:
    • Level 3 Boss: Kamikirimusi
    • Main Boss: Siegfried
29. Voldo:
    • Level 3 Boss: Scheherazade
    • Main Boss: Nightmare
30. Xianghua:
    • Level 3 Boss: Kamikirimusi
    • Main Boss: Algol
31. Yoda:
    • Level 3 Boss: Scheherazade
    • Main Boss: Algol
32. Yoshimitsu:
    • Level 3 Boss: Scheherazade
    • Main Boss: Algol
33. Yun-seong:
    • Level 3 Boss: Kamikirimusi
    • Main Boss: Nightmare
34. Zasalamel:
    • Level 3 Boss: Angol Fear
    • Main Boss: Algol