Marvel: Ultimate Alliance

Hint: Play as Iron Man:
Fully complete (through level 10) two of the S.H.I.E.L.D. simulator missions.
Information in this section was contributed by Alex B.

Hint: Play as Silver Surfer:
Fully complete all four S.H.I.E.L.D simulator missions.
Information in this section was contributed by Alex B.

Play as The Thing:
Play the Omega Base level. After talking to Dugan, enter the door to his left and defeat the enemies. Then, go as far up against the wall as possible. Walk to the right, and you will eventually see a doorway. Go inside to find The Thing and Bruce Banner. Talk to them and the "Character Unlocked" message will appear. You can now select The Thing during the next mission.

Hint: Striker combos:
Summoning certain Striker characters with specific characters will yield different results:
Information in this section was contributed by Alex B.

Summoning Dr. Strange with Blade makes him generate a demon animal which does much more damage.

Summoning Namor with Miss Marvel makes him turn into a shark, doing much more damage.

Summoning Iron Man with Captain America makes him put a shield around you.

Summoning Phoenix with Iceman makes her drain your health.

Ghost Rider and Silver Surfer do not appear to have any combo partners.