Font rendering is pretty complex, so many video games leave it out. Given that, and that the Gamecube had to render to a 480p TV screen, I would be very surprised if they used a real font instead of a series of square bitmaps laid next to each other. Further evidence: they're all the exact same height, the antialiasing looks hand-drawn, if you look at a full screenshot of the Character Select Screen, you can tell that there is no regular spacing between the letters- it depends on how many are letters are being squeezed into a line. So, if you took the images above from a full-resolution screenshot, you have everything there is to the 'font'.
That was one hell of a game though.