Hotaru (whose original name was Keikoku) was a member of the five stars of the Mibu and was sent to spy on Kyo. He has great admiration for the blood-thirsty and icy Kyo and resents Kyo's softer side that has developed by coming into contact with Yuya. He also resents his older half-brother Shinrei (another member of the five stars) due to the fact that Shinrei is the valued "wanted son" while he, Hotaru, is the illegitimate son of a concubine.
Despite Shinrei's ignorance of the truth, Hotaru's entire life seems centered around his animosity towards his brother. All he does is counter Shinrei. Where Shinrei uses water (their father being a Master of the Muryou Saikyou Ryuu School, the water school of the Mibu), Hotaru devotes himself to fire. Where Shinrei swore to uphold the honor and wishes of the Mibu Clan, Hotaru rebelled. Where Shinrei sought strength through friendship and camaraderie, Hotaru sought strength through solitude. This prompts him to battle his former allies, the Shiseiten and Kyo, because he believes that they stand between himself and the solitude needed for true strength.
Hotaru is defeated by Kyo and becomes his "servant" as his life was spared by him. He stays behind in order to recover from his injuries but reaches Kyo and his friends when Kyo is about to be defeated by Shinrei. Hotaru battles his half-brother and, much to his surprise, discovers that Shinrei already knew they were related. Moreover, Shinrei was the one who convinced their father to stop sending assassins after Hotaru and even managed to get Hotaru accepted as one of the five stars of the Mibu, mainly because Shinrei wanted to face Hotaru as an equal since Hotaru was his only brother.
Hotaru is defeated by his brother, who merely puts him unconscious while pretending to have killed him. When Shinrei is about to be killed by Kyo, Hotaru protects him and pleads to Kyo to spare him. Hotaru later rejoins the Shiseiten to battle his teacher, Yuan, and the rest of his former clan.
Hotaru's command of fire allows him to summon black flame, which is hotter than any normal flame, and to generate a solid shield of flame to protect himself. He reveals many techniques, among which include: Chikewai (Hotaru applies blood on his face, increasing his power), Shakuran Entai (a blast of fire that covers a large area), Kaho Enbu (traps a foe in a shell with flames burning inside; when the foe attempts to break out, it causes a massive backdraft), Ma'oh-en (summons a bigger and hotter flame than Shakuran Entai), Demon's Breath (summons black fire and shoots it at his enemies), and the Hell Crusher (surrounds himself with a massive field of black flame leaving a huge crater). In the battle against his master, Yuan, he reveals his ultimate technique, Keikokuken [Hotaru creates a maelstrom of fire which surrounds his opponent and pastes Chikewai on his opponent which burns them to death (this technique can also apply Chikewai on his partner to increase their skills)]. He fights with a double-sided sword: one side shaped like a katana, the other shaped similar to a long dagger. It is believed this sword is just one long blade with wraps and a hand guard so he can hold it.
Hotaru is the comic relief of the group. He act extremely stupid and dense and always states the obvious. Some examples of this are Hotaru's fight with Kyo (he makes a long speech then forgets what it was suppose to be about), when he leads Kyo's group through the Mibu palace (When asked directions, he says they should go straight when the path ahead of them is a straight one), his battle with Yuan (When Yuan said he can beat Hotaru with only his foot, Hotaru said he will beat Yuan with only his hands), and various others.
Hotaru, after realizing that he was not alone in the world, became deeply concerned with protecting and saving the ones he cares about although he doesn't show it directly. He calls Kyo "a friend of combat" and tries his hardest to protect him. He also becomes friends with Benitora, Sasuke, Yuya, and Yukimura. He is considered a family member by Yuan's family.
Also, his relationship with Shinrei becomes much more close. Although they both claim to still hate each other, their fights and teasing are much more similar to sibling rivality. Throughout the manga, Hotaru keeps saying that he hates water and continuously teases his older brother; however, he has shown that he deeply cares about him and will save and help him whenever he can.