(Note: This guide is provided only to identify secret achievements. I'm afraid that I couldn't answer any question about this guide.)
Rimi made her appearance!
There are two ways to make Rimi appear in the game.
1. Fight with her in online versus play. You don't have to win the match.
2. Finish the story mode of Fin.
The first would be easier. But either way, you must finish Fin's Story mode if you want to complete this game.
See the following achievement for how to unlock Fin.
Note that to use Rimi as a player character, you still have to unlock her. See the guides for the following achievement.