Tomoe Tatsumi is the younger brother of Souichi Tatsumi and the older brother of Kanako Tatsumi, and is married to Mitsugu Kurokawa. He moved to Tokyo for university and there met Mitsugu Kurokawa, an office worker captivated by Tomoe's helpless and naive demeanor. Facing financial difficulty, Tomoe accepted Kurokawa's offer to become his sub-lessee.

Kurokawa confessed his feelings and kissed Tomoe, leaving Tomoe both confused and irritated, though he seemed to be developing reciprocal feelings. Their relationship continued to grow despite opposition from friends and relatives. Tomoe's brother Souichi, distrustful of the living situation and holding homophobic views, tried to convince Tomoe to move back home.

Tomoe is highly intelligent, especially in the field of robotics, yet he is also ditzy, absent-minded, naive, and somewhat helpless—a direct contrast to his older brother's violent and quick-tempered disposition. Tomoe is generally quiet, shy, and cute in manner.

After gay marriage was legalized in California, Tomoe and Kurokawa relocated there for Tomoe's new job and eventually married. This decision infuriated Souichi. Unfazed by his brother's anger, Tomoe remains determined to live independently and has expressed a wish to talk openly about his marriage, though Souichi consistently avoids the subject.

Tomoe keeps in regular contact with his family from America, often calling to soothe and reassure his brother Souichi. Their father, Soujin Tatsumi, was initially shocked by news of the marriage but later accepted it, affirming that Tomoe's happiness was most important.