Tachibana no Tomomasa holds the position of Byakko of Earth (地の白虎) among the Eight Guardians (Hachiyou). A nobleman serving as a general in Kyou, he holds the rank of Lieutenant of the Left Imperial Guard and occasionally acts as an imperial messenger.

He presents a flirtatious and nonchalant demeanor, charming women with ease and enjoying popularity among them. Beneath this carefree exterior lies a sharp mind and a strong ability to judge character accurately, though he frequently conceals this perceptiveness. His elemental attribute is Swamp, and his dragon jewel, the source of his Guardian power, rests between his collar bones.

His initial interactions with the protagonist, Akane Motomiya, involve playful teasing and flirtation that makes her blush. Over time, his feelings deepen beyond amusement; he becomes genuinely enchanted by her kindness and smile, developing a protective affection. This progression culminates in his personal storyline where he openly reveals his true feelings and expresses a desire to stay by her side.

Within the Eight Guardians, he maintains cooperative relationships with all members: Minamoto no Yorihisa, Tenma Morimura, Fujiwara no Takamichi, Abe no Yasuaki, Shimon Nagareyama, Eisen, and Inori. He is also aligned with Fuji-hime and Kotengu. His primary adversaries are members of the Oni Clan, specifically their leader Akuram and the oni Shirin.