Leonidas is the main bakugan from the game Bakugan Battle Brawlers. He was originally created from the dark emotions of the bakugan trapped in the doom dimension, and merely wanted to take out his anger on other bakugan. The only way to do so was to team up with a human that could unleash his power, which he almost immediately did after birth.
The player finds himself in a flashing light at the beginning of the game, and after the light dies down Leonidas appears next to the player. He states his name and the player asks to be his partner; Leonidas agrees, seeing this is the only way to unleash his fury.
Immediately after their first tournament, many of the Battle Brawlers grow suspicious of Leonidas, wondering why he is so aggressive in battle and why he came to earth at such a late time. The player shakes it off, stating Leo is his partner, and he will not simply throw him away. After this, Leonidas begins to feel trust and friendship, making him want to turn away from his aggressiveness and hate. He eventually does so, after he reveals his origins to the player. He states he has begun to feel trust, and he is sorry for keeping secrets. The player, however, thanks Leonidas for being his partner, and the two continue entering in battles.
Eventually a Bakugan called Vladitor rises to power and challenges Leonidas to a tournament. Leonidas and the player win... however, Vladitor seized the "silent core" and used it to become incredibly powerful so he could destroy all bakugan-Leonidas included. Leonidas, however, takes the powers of his friends and uses it to evolve as well. He defeats Vladitor, and the two fade due to the overload of power they used.
Leonidas reappears, however, stating that Vladitor saved him at the last second, stating he deserved to live.
Leonidas can change color depending upon his attribute, but his physical features always stay the same. Despite this, his card always shows him being Haos (light) despite him being born of darkness. He bears a strange resemblance to dragonoid.