Ichi Tenma, also known as Oichi, has long dark blue hair and brown eyes, and she often wears an orange star clip in her hair. Her typical outfit is a blue dress with a yellow bow and a white jacket.

She maintains a very cheerful nature while often seeming to know more than she lets on. She holds a position of power as the youngest executive in the International Battle Spirits Association. Arriving in Musashi to observe secretly without revealing her identity, she met Yukimura Rekka and his companions. Noticing he possessed Muramasa-Dragon, she expressed a desire to acquire it and revealed she had a Soul Burst card that would complement it. She orchestrated a battle between Yukimura and her bodyguard, Katsuie Shiraishi, with the two cards as the stakes. Although Yukimura won, it was later revealed she had always intended to give him the card and had arranged the battle to make the transaction more interesting.

Later, she met Souun Gunjou, who was discouraged by constant losses. Oichi inspired Souun to love the sea again and also gave her a Soul Burst card. She announced that the IBSA would host the first-ever national Battle Spirits tournament, the Battle Spirits Warring States Championship, and she watched the matches as they progressed.

It was later revealed she is the younger sister of Nobunaga "Nobu" Tenma, the eldest child of the Tenma household and the former mentor of Yukimura Rekka. She supported Yukimura in his battle against Nobunaga to stop his plans. At the end of the series, she and other IBSA members bid farewell to Nobunaga as he decided to leave Japan to begin a journey with Toukichirou Mashira and Ranmaru Shikigami.

In a separate account, a fanfiction story proposed an alternative scenario where Ichi and Nobunaga had a brother named Tenma Hidetaka, but this is not part of the official character background.