When you have completed the Great Robo Cup the following Robos can be unlocked by completing the corresponding task.

Unlock Carat:
Beat ALL limited Robo missions and then defeat Angelica.

Unlock Hadron:
Complete ALL four Greyham Boss data and Hadoron data to unlock Hadoron and his parts.

Unlock Illegal Robos:
At the Underground go to the middle door and talk to the clerk selling parts.

Unlock Ray 01:
At the Underground go to the middle door and talk to the clerk selling parts.

Unlock Ray Sky:
At the Underground go to the middle door and talk to the clerk selling parts.

Unlock Hiykko:
At Team No. 1's Clubhouse talk to the left box above the stairs. Then defeat the kid that is hiding inside the box without having a combo over 380 damage.

Unlock AIRS:
Defeat Stark twice in the grudge matches and AIRS parts will be available in the Underground Shop. Stark will use his regular Robo the first time you battle and then use AIRS the second time.

Unlock Majel:
When you have defeated 12 characters in the grudge match go in front of the clubhouse and fight your shadow.

Unlock Ray II Dark Robo:
Defeat your shadow self in the underground for the second time.

Unlock Mistral:
On the final grudge match go to your clubhouse and defeat Eddy.

Unlock Ray and Sagittarius Gun:
Defeat Saki in your clubhouse after she returns.

Unlock Ray MK II:
Go to your father's lab in NeoBrain and talk to him.

Unlock X-Ray:
At the Police station talk to the person near Kris on the left sitting on the table and he will ask you to purchase it for $500 credits.

When you get to King Rank there are 2 hidden bonus dioramas (King's Throne and Victory) which can be unlocked by talking to Lambda Halburn at Neobrain during the Grudge Battles.

Win aproximately 12 Grudge matches and defeat your shadow self. Then talk to the man in the south west Bayside area to open the secret entrance.

When you have successfully beaten the 'Robo Cup' find the scientist facing the glass in one of the laboratories on the upper floor of NeoBrain and then defeat him. You will then be able to purchase the stage.