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