Update: Turns out I was wrong. I found out from a friend of mine, Piyochan666, that Midori and Akane appear in Alternative Game, a mini-game you unlock after beating the Caribbean Course. Your opponents are always Gene & Akane, with Midori being your partner.

Since I played the original Japanese version - before the fan translation happened - I wasn't aware of that. I do vaguely recall translating the "you unlocked alternative game" screen, but I didn't know what that was. (I always called it doubles, since Advance Tour does so.)

That being said, the game still crashes after you win/lose if you access it via the Debug Menu. Playing it the normal way doesn't do that.

For anyone wondering, Midori and Akane are two NPCs that were part of an online event/challenge in the game. As the text says up above, you partner up with Midori while Akane partners up with Gene Yuss. (whether she could partner up with anyone else, I don't know)

You can still access it by activating the Debug Menu and going to Room 13. Talk to the Toad at the lowest part of the room, and you can play the event. However, regardless if you win or lose, the game crashes after going through the final results screen, probably because the game is assuming you're online and when it finds out you aren't, it doesn't know what to do.