Reposted from another comment
The more overt choices to where the storyline branches are in the Sportsdome - responding to Lanks or Cyclops in a certain fashion will change the course of the story.
MAJOR SPOILERS (In case you want direct instructions):
To unlock one ending, respond to Lanks by picking all of the TOP (first) options in dialogue for EACH match. (Four times)
To unlock a different ending than that, respond to Cyclops by picking all of the BOTTOM (second) options in dialogue for EACH match. (Four times)