"Meet Flea, the magician"

First encounter

We meet Flea for the first time at Magus Castle in 600 AD. He waits for you in the east wing of the castle. Of course he is using some magic to fool the team. The character who defeats the fake Flea will be drained of all MP.

The tiny bat, which has secretly been following the party since the magic cave, now reveals itself to be the true Flea. He is resistant to magic attacks and has to be attacked with physical weapons. He will cast chaos spells that either change the status of a character (sleep) or bewitches them, so that they are unable to control their attacks.

Stats: HP: 4120; Exp: 500; Tech: 10; Gold:1000;
Weakness: physical attacks; Strong against: Magic

Second chance: Flea Plus

On a side quest you can defeat Flea for a second time. After Magus disappeared into the past, Ozzie and his cronies moved over to Ozzie's Fort. But the battle at Magus' Castle must have weakend Flea a lot. He's less strong and can now easily be defeated with magic.

Stats: HP: 2500; Exp: 2000; Tech: 20; Gold:2000;
Weakness: none; Strong against: none

Great Ozzie, Super Slash & Flea Plus

The fearsome three finally unite in one last battle. Together they can cast powerful double & triple techs. Flea also wears his special Flea Vest/Bustier, which can be stolen by Ayla.

Stats : HP (Flea): 4000; Exp: 7500; Tech: 80; Gold:4000;
Weakness: none; Strong against: none; Charm: Flea Vest/Bustier

...Back to the future

Once the Fearsome Three are defeated, mankind and Mystics can finally live together in peace. The statue of Ozzie at the square in Medina Village of 1000 AD has never been build, Ozzie's grand-grand-children clean the kitchen of the Major's house (that's... an improvement somehow?) and mystics no longer hold a grudge against humans.
We never learn what happened to Flea and Slash or their offspring.

Chrono Cross

There's not much to say about Chrono Cross, but here are Flea's stats from the secret battle:

Stats: HP: 1200; Attack: 55, Magic: 20, Defense: 43, Magic Defense: 16, Element: Red,
Other Elements: Turn White, Turn Black, Turn Green, Turn Yellow, Turn Blue, Turn Red, Fireball, Weaken, Fire Pillar, Strengthen, Anti White, Anti Black, Anti Green, Anti Yellow, Anti Blue, Anti Red, Inferno, Ninety-Nine, Omega Series, Volcano
Innate: The Stare, Description: Damage & confusion (All Enemies)
Weakness: Physical attacks; Strong against: Magic
Drop: Dreamer's Sash: start off with element level 2 in battle, Steal: Winged Shoes: Evd%+18 (Common), Flea Vest: M.Def+13 (Rare)