Load a game after completing it. Choose Select from the character screen, then ARMS. Complete the game again to gain more outfits.
There are two endings to the game, both can be triggered after the cutscene where Rachel stops the countdown by releasing her energy. After you see that, you need to reach the basement and last boss in less than 10 minutes to get the good ending. If you take more than that, you'll see a cutscene where Rachel dies just before you fight the last boss, and that will net you the bad ending.
Each of the four main characters must fight their rival boss.