Rena Asagi stands approximately 5'1" tall, noted for her short stature. Her distinctive appearance features a protruding tooth, and she is often seen with a ponytail and hair ribbons. Her hair color lies between auburn and brown, though sometimes appears violet. Similarly, her eye color is vividly pink in some instances and aubergine in others. Her figure includes a noticeably large bust, explicitly described as an "explosive chest" within the series and noted as comparatively larger than another female character's.

Rena displays overt extroversion and confidence, frequently engaging in mischievous or naughty behavior. Despite this outward demeanor, she possesses significant intelligence, having ranked as the top student in her middle school class. She demonstrates pragmatism in work, openly offering to perform odd jobs for payment, while explicitly excluding sexual services. This transactional approach stems from her financially disadvantaged family background.

She comes from a poor family and attended the same middle school as Tetsuhiko Kai. A later familial revelation discloses that she and Tetsuhiko are half-siblings, resulting from one of their father's affairs. This connection leads Tetsuhiko to help her secure employment; he assists in founding the Group Blue Alliance primarily to provide her with a managerial position in exchange for appropriate compensation. Rena is initially introduced to the protagonist while assisting Tetsuhiko with pizza delivery to their class, presenting herself as a "Jack of All Trades" available for paid tasks.

Her given name "Rena" combines the characters for "tinkling, exquisite, clever" and "vegetables, cuisine, dish." Her surname "Asagi" translates to "light yellow," following a series naming pattern where female characters' names incorporate color references, with "Asagi" representing yellow.