Finish the game and select the new game + and rescue the twenty five cats of the game, to get the pin cat, this gives you infinite SP.
Hair Ribbon
See the true ending on the game.
Kunio and Riki
Finish the game once to unlock the original River City Ransom Team, Kunio and Riki, Both have the powerful Sou-Setsu-Ken techniques of the original double dragons Billy and Jimmy Lee, used in the prequel game Shin Nekketsu Koha: Kunio-tachi no Banka.
New Game+
Beat the game once, then select the new game +, in this mode you can carry all your money, items and techniques obtained during the game.
Secret Store
Go to the very end of the slums, the tunnel, before the entrance to the beach zone, it's located in the upper middle of the tunnel, and it contains two secret items with each cost $5000, one grants you insane attack and the other gives you insane speed.
True Ending
To get the true ending, you must break the twenty five man statues throughout the game to obtain mami's charm, then beat the final boss saboku to obtain Hasabe's charm, then, equip both medallions and enter the saboku room again, to see the true ending.
Various Steam Achievements
Complete the indicated task to unlock the achievement.
Achievement
How to unlock
A Bye Bye
Unknown.
A New Challenger
Unknown.
A Real Workout
Unknown.
An Explosive Entrance
Enter the Penthouse.
Back at me!
Catch a boomerang.
Bona fide bargain
Obtain an item from the Pawn Shop.
Carpe diem
Unknown.
Completionist
Reach 100% completion.
Defeating the Dark Arts
Unknown.
Don't mess with us!
Complete the game in Co-Op.
Fashionable Beatdown
Unknown.
Friendly Beatdown
Defeated an enemy by hitting them with another enemy.
Cat Pin...