These are the characters that either made only brief appearances, or was portrayed as a hero in one medium and foe in the next.

Real Name: Tabitha Smith

Powers: can create energy plasmoids, or time bombs, from her bare hands of any size or duration.

Bio: Spunky and wild, Tabitha is carefree and laidback.  Her father once used her powers for selfish means, but she bounced back.  She stayed with the X-Men for a time, and after a new incident with her father caused some trouble, she moved in with the Brotherhood boys until Mystique came back. Afterwards eventually went back to the X-Men.

Romantic Interets: On Evo she was a great big flirt, but never focused on one man for long.

Nemesis: Her father

Trivia: Boom Boom eventually changed her codename to Boomer and became romantically involved with Cannonball. Unfortunately, this romance was never even hinted on X-Men: Evolution.

Real Name: Callisto

Powers: Enhances senses including night vision.  She is also a skilled fighter. In X3 she also has Quicksilver's superspeed and Caliban/Sage's ability to locate mutants.

Bio: As leader of the Morlocks in the cartoons, and persumably the Omega mutants (or mutants with the Omega symbol on them- who knows what was going on with some of the people in that movie) in X3, she does what she has to do in order help other mutants survive.  When Storm defeats her in TAS she becomes resnetful of her, and in X3 she repeatedly initiates fights with Storm before Storm finally gets annoyed and fries her.

Romantic Interests: In TAS she wanted Cyclops to rule the Morlocks as her king and provide her an heir.

Nemesis: Storm

Trivia: Not only was she granted more powers than she should have had in X3, but Callisto also lost the eyepatch and scared features that defined her as a Morlock.

Real Name: Alexander Summers

Powers: Discharges plasma blasts from his hands created from optic energy that can superheat or expell a massive impact.

Bio: On TAS he is a member of X-Factor and has no clue his brother is alive.  On Evo, Alex and Scott are reunited and Havok eventually joins the X-Men at the end of the series presumably.  Alex also has his last name changed from his adopted surname: Masters, back to Summers.

Romantic Interests: Polaris

Nemesis: Cyclops (TAS)

Trivia: Cyclops and Havok are immune to each other's powers.

Real Name: Longshot

Powers: Longshot can control and manipulate probailities in order to always have "good luck".

Bio: Created to be a star in Mojoverse, Longshot later rebels against Mojo.  He helps the X-Men excape Mojoverse in order to regain his timeslot, but later somehow ends up leaving his dimension and having to help save Jubilee from Mojo.

Romantic Interest: Spiral - although it clearly didn't work out since she tried to kill him the second time Longshot appeared on the show.  He formed a bond with Jubilee as well.

Nemesis: Mojo

Trivia: Longshot was at one time married to Dazzler, and she appeared with him in "Mojovision".

Real Name: James Madrox

Powers: Using kinetic energy as a catalyst, he can create multiple exact duplicates of himself.

Bio: He was a member of X-Factor on TAS, a new recruit for the X-Men in Evo, and part of the brotherhood in X3.  Flexabilty much?  He is the youngest recruit in Evo, and there for always scoffed at by the older students as beig in the way and taking up too much room (which he does when he multiplies).  In X3 he was arrested for robbing 7 banks at the same time, and is therefore on the mobile prison Mystique is held on when Magneto finds and recruits him.

Romantic Interests: N/A though he is rather fond of Kitty in Evo when she has to take him as a date last minute

Nemesis: N/A

Trivia: The symbol on his shirt in X3 is the actual Multiple Man symbol worn by him on his uniform in the comics. Also, because he is so young on Evo, they dropped the "man" from his codename and he goes by "Multiple."

Real Name: Lorna Dane

Powers: Similarly to Magneto, Polaris can use magnetic energy and use as pulse bolts, a means to fly, create force fields, and control some ferrous materials at her will. 

Bio: Not too much is known about Polaris from her one appearance in TAS.  We know she was an X-Man alongside Iceman and left when he did, and then left Bobby to join X-Factor where she met and fell in love with Havok.

Romantic Interests: Iceman and Havok

Nemesis: N/A

Trivia: Even in the comics, Lorna is constantly having to decide between Alex and Bobby.  She also became Apocalypse's Horseman known as Pestilence, and Alex took all the germs from her to breathe life back into her while Bobby stood by and watched.

Real Name: Elizabeth "Betsy" Braddock

Powers: Telepathic, with the ability to project telepathic power into a weapon - most notably her psi-knife.  In X3 she also has the ability to fade into her surroundings.

Bio: Psylocke has a very complicated history, so lets stick with what we know of her in TAS and X3. In TAS she is trained as a ninja, but she using her stelth skills to steal for her brother, Captain Britian for good causes.  In X3 she is part of Callisto's little band of mutants.

Romantic Interests: An attraction to Archangel appears in TAS, though she fights him anyway.

Nemesis: N/A

Trivia: Also cameoed in TAS episodes "Repo Man" and "Mojovision"