How do I unlock more characters on Dragonball Z Budokai Tenkaichi 2 on PS2?

I've completed the main story mode and am currently working through the Battle Z challenges. But how do you unlock the other characters? There are still a lot of '???' on the character select bit. ANy help would be great. Thank you :)

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).