To acquire your character's True Ending, you need to collect a specific amount of Ability Points before you reach Stage 8.
The required amount varies depending on your Round Settings:

• For 1 Round, you need 70,000 points;
  
• For 3 Rounds (first to 2), you need 95,000 points;
  
• For 5 Rounds (first to 3) you need 110,000 points

All of the True Endings operate the same way: Your regular ending plays out, but instead of the game progressing to the credits, you'll instead be challenged to a Boss Fight. Beating the Boss reveals the True Ending.
Don't worry about losing, you can continue and fight the Boss again. You'll be locked out of changing characters, however.


                                             IMPORTANT NOTES:

• It doesn't matter if you lose and use a continue, so long as you collect enough points.
  
• Also, even if you think you don't have enough points by the time you finish Stage 7, but are somewhat close (let's say you chose 3 rounds and have 72,100 points), try and fight Bilstein anyway, as defeating him gives a huge score bonus, that just might get you in the threshold necessary to get the True Ending. Since you don't see a regular victory screen after defeating Bilstein, you'll have to watch the ending and wait to see if the text "Here comes a new challenger" shows up on screen.

To face off against Kaede or Rai-On, you must meet the following conditions, before the 4th match:

• Get at least 2 Perfect victories;
  
• Get at least 3 Plasma Strike finishes;
  
• Get at least 1 Plasma Field finish;
  
• Don't win any matches by Time Out;
  
• Don't use a Continue (you can still lose a round, however)

If done correctly, you'll run into either Kaede or Rai-On at random, as the 4th match.

The game doesn't put much fanfare about the fight at all. You'll just see the regular "Stage X" screen with a portrait of either Kaede or Rai-On, as if they were a common opponent.

To unlock the achievement "The Smaller they are..." simply choose Luca or Gore and wait for the round to start. Then input the following combination:

Down, Down, Forward, Back, Start

(The in-game hint and the move list both say to do Forward and Back twice, but it's not necessary, as highlighted by fellow user Azriel Arcade)

Alternatively, you can simple press down and the Instant Special Move button (LB button by default) to execute the special move.

The character has to successfully finish the animation for it to count.

If you hold Start before selecting Gamof, you'll be able to play as the evil version from his story!
It's simply a skin however.