Takeru Aizawa is the younger brother of Eiko and Chizuru Aizawa. He attends elementary school alongside his friends Yuta Matsumoto, Shota Kuroishi, and Satoshi, with Aiko Saitō as his teacher. Physically, Takeru sports black spiky hair and black eyes. He typically wears a green t-shirt and brown shorts, distinguishing him as the only Aizawa family member who doesn't wear long-sleeved pants. Ika Musume initially dismissed Takeru and held a low opinion of him. Undeterred, Takeru consistently sought her out for play. Over time, Ika Musume's view softened; she grew to see him as a friend and even adopted a third older sister role towards him. She often involves Takeru in her schemes, occasionally using him as a test subject, yet also demonstrates genuine concern for his well-being during their interactions. Takeru displays a notably childish demeanor appropriate for his age. His persistent efforts to befriend Ika Musume showcase his friendly and approachable nature. He affectionately calls her "Ika Nee-chan" (Big Squid Sister), reflecting their close bond. Takeru primarily provides Ika Musume with companionship and opportunities for play, frequently acting as her main playmate among human children. This dynamic aids her gradual integration into human society and tempers her initial world domination ambitions. His consistent presence reinforces the narrative's themes of friendship and adaptation.