Sent in by Jeff Swanson
After you win everything (Stadium and Gym Leader Castle) you will
see the credits like normal but then you have a 6-on-1 battle with
Mewtwo (Psychic, Seismic Toss, Thunderbolt, and, of course,
Recover). When you beat him you unlock R2 Mode.
Gym Leader Castle Ending: It does the reg. end battle stuff and then
it goes through more stuff and you get Eevee, Bulbasaur, Charmander,
Squirtle, Kabuto, or Omanyte.
Stadium: Same thing as Castle but instead of a Pokemon, you get a
cup. get Masterball on each one to obtain Doduo mode (twice as fast
(only requires one)) and Dodrio mode (3x as fast) for GB tower.