Here is the full list of all 29 River City Girls achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Reach 100% completion.
Was defeated in detention.
Completed the game with a secret character.
That wasn't very nice!
Reached Max Level.
K.O. the secret boss.
Defeated an enemy with a fish.
Catch a boomerang.
Parry 10 enemy attacks.
Equip a new outfit.
Destroyed all Sabu statues.
Catch a dodgeball thrown by your ally.
Complete the game in Co-Op.
Collected $5000
K.O. Sabuko.
Completed the game on Hard.
Enter the Penthouse.
Completed all of Godai's quests.
K.O. Noize.
K.O. Abobo.
Find the keys in the junk yard.
K.O. Hibari.
Obtain an item from the Pawn Shop.
K.O. Yamada.
Enter the Mall.
Defeated an enemy by hitting them with another enemy.
K.O. Misuzu.
Attacked Mr. Rudis.
Destroy a vending machine.