Kurumi Tomonaga is the middle child of the Tomonaga family, the second daughter of Yuuichirou and Amami Tomonaga. She is thirteen years old, younger than her older sister Azuki and older than her younger brother Yuuta. Kurumi is a tomboyish, impatient, and hot-blooded girl who thrives on being the center of attention. She can become jealous when others draw the limelight away from her, as happened when the press swarmed over her younger brother.

Cheerfully abrasive and fond of gossip, she constantly teases Yuuta and bickers with him, but beneath the arguments she holds a deep, genuine care for her sibling. Her role in the story is that of a supporting family presence, adding a dose of everyday domestic life to the high-tech action. While she does not drive major plot beats, her interactions with Yuuta, Azuki, and eventually the robot ninja Shadowmaru highlight her loving if sharp-tongued side, and she becomes a friend to Shadowmaru alongside her sister by the series' end. Kurumi has no special powers or combat skills; she is an ordinary middle-school student with short neck-length brown hair and blue-green eyes, notable for her spirited personality and the warmth hidden beneath her teasing exterior.