So after playing and watching some offsets of Mario's Tennis for the Virtual Boy, I made a rather fascinating discovery. Take a look at the character select screen here:
(Pardon the glitches, the emulator is incomplete.)
However, when in comparison to its hexadecimal equivalent, there's a notable string.
Starting from 0x7D694, there's a list of character names, all in the same order; Mario, Luigi, Princess, Yoshi, Toad, Koopa and DK. There's an extra name at the right, Cassarin, which apparently translates to Birdo. (Thanks, Chup)
What might this mean? I've read some FAQs and I see no mention of this
"Cassarin", or Birdo for that matter at all.