Niina Isokawa is Marie Kagura's energetic classmate. Born to a Japanese father and an American mother, she has distinctive blonde hair. Niina bubbles with restless energy and an exceptionally friendly personality, unable to sit still. She immediately attaches herself to Marie upon meeting her, though Marie initially finds Niina's persistent overtures overwhelming and expresses displeasure. Niina explains her intense interest is inspired by Marie's resemblance to her cherished plush toy, Raku-kun—a name likely referencing a raccoon despite the toy resembling an otter.

Despite her youth, Niina is notably busty. This physical trait becomes a point of internal conflict for Yuji Kagura as Niina develops a precocious crush on him, often calling him "onii-chan." Yuji feels torn between attraction and awareness of her being underage.

Niina's character development centers on her relationship with Marie. Her initially overbearing behavior causes Marie significant distress, especially due to Marie's introverted nature. A turning point occurs when Niina saves Marie from a group of cats, which Marie fears mortally. This act, combined with Yuji explaining Marie's perspective to Niina, helps Niina understand Marie's boundaries. Following this, Niina gains acceptance and becomes a more integrated part of the group surrounding the Arisaka and Kagura families. Her persistence evolves into a genuine, less one-sided friendship with Marie.