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