Leonidas is the main Guardian Bakugan of Valentin Kazami. He is the only Bakugan existence that is always tied to Drago in power. He is also Drago's Darkus counterpart.
Being Leonidas' fiancee, Josette tends to flirt with him, and acts like a teenage girl around him. Josette only listens to Leonidas, and deeply cares for him, even tending to him when ever he is upset or feeling guilty. Leonidas returns these affections by caring for Josette in return, and will do almost anything to protect her and keep her for himself. This is especially when Josette goes beyond her limits in power in battles, which causes Leonidas to be extremely concerned over her safety.
Ever since Spade found out that Leonidas became Josette's fiance without Spade himself knowing, his psychotic mind had became more hectic, and he vowed to kill Leonidas and keep Josette to himself, and kill Josette if he did not succeed. However, he was killed by Leonidas, who had reluctantly committed the deed in order to protect Josette.
Leonidas was created by Naga with the help of Hal-G in the Doom Dimension, resulting him in being the first Bakugan being born in the Doom Dimension ever. He was sent to Earth to track down Drago and destroy him, but Leonidas had no memory of who he was, other than that his name was Leonidas.
Valentin had discovered Leonidas sitting on the marble part of a fountain in the park in Wardington. He asked Leonidas who he was, and later Leonidas and him had a small chat. Leonidas then agreed to go with Valentin, who need a partner. The two then battled Julie, Marucho, Runo, and Shun when they suspected him and Leonidas of badly taking down Dan and Drago, though the guilty ones were really Marduk and Vladitor, Valentin and Leonidas' main adversaries at that time. When Valentin and Leonidas beat them all, they proved that they were Dan and Drago's friends, and would have never hurt them. They later battled numerous opponents, such as Masquerade and his vicious Hydranoid, Chan Lee, Julio, Jenny and Jewls, Klaus, Billy, Komba, and even Marduk himself. The two friends one all of their battles at that time.
When Marduk and Vladitor talked to Naga and Hal-G about their mission to retrieve Leonidas, they told the evil beings that they were met with failure. Vladitor, however, demanded the Silent Core Orb promised to him by Naga - if he and Marduk completed the mission. Naga refused to hand over the orb, and the orb was stolen and absorbed by Vladitor, causing him to evolve into Battle Ax Vladitor.
Later, Marduk and Vladitor challenged Dan and the evolved Drago, defeating them easily. They did the same to the other four original Battle Brawlers, whose Bakugan had evolved. Valentin and Leonidas were told of this and began to prepare for a final confrontation between Marduk and Vladitor.
As predicted, the two villains challenged the two heroes, but before the Battle Brawlers and their Guardian Bakugan gave Leonidas some of their new power, causing his to rise up to an extremely high level. This also resulted in his first evolution - Omega Leonidas (which was originally intended to be Leonidas' only evolution, as "omega" represents anything that is final).
Vladitor, Marduk, Valentin, and Leonidas battled it out. The heroes won, but at a cost - Leonidas was supposed to die alongside Vladitor, for it was their destiny. As the two Bakugan died, Leonidas told Valentin to never forget him and remember their good times. Valentin nodded and promised he would remember with honor and virtue.
When Marduk began to sob, Valentin, who took Leonidas' death "as a true man", told Marduk to accept the fact that Vladitor was gone.
The next day, at the same spot where the two met, Leonidas and Valentin were reunited as good friends. Leonidas explained to Valentin that Vladitor had given up his a portion of his remaining power in order for Leonidas to live, for the once-evil Bakugan was truly honored by Leonidas' decision to die and his bravery to face it. Valentin and Leonidas later brawl Dan and Drago once again as part of a celebration.ed
Invasion of the GatekeepersEdit
When Naga's Gatekeepers invaded the Earth, Leonidas and Valentin were first called to the scene. They clashed with all six when they were united, and amazingly managed to defeat all six, but only temporarily, despite it being enough time for the Battle Brawlers to return to Earth from the damaged and disorted Vestroia. The two later battled Naga as he came to Earth, and would have died if Wavern had not intervened, transporting the two back to Wardington. The two later battle Naga once again. When Naga reveals that Leonidas was really his creation, and the intended seventh Gatekeeper, everyone is afraid of Leonidas. However, Dan and Drago help Valentin remind everyone else the good Leonidas did. Leonidas than vowed to help kill Naga, which caused everyone to understand that he was still good. He later keeps his promise by helping the other Battle Brawlers' Bakugan give Drago their energy to destroy Naga (or supposedly send him to the Doom Dimension).
Bakugan: New VestroiaEdit
Leonidas had left with the Bakugan for New Vestroia in the first season in order to see New Vestroia for himself and protect it, despite not being born in it. Three years after the events of the first season, and days after Drago took Dan to New Vestroia, Leonidas located Valentin through New Vestroia and transported him there, and met up with him. Leonidas informed Valentin about the recent events in New Vestroia, including the invasion of the aliens known as the Vestals and the enslavement of the Bakugan in their hands. Valentin agreed to partner up with Leonidas once again in order to free the Bakugan and save New Vestoia. The two later discovered an ancient ruin, and after investigating it, had to battle Shadow Prove of the Vexos. The two easily won, however, and obtained Shadow's Bakugan.
More to be added.