Iku Sutō is a first-year high school student and the ninth girlfriend of Rentarō Aijō in the anime and manga series. She is a student at Ohananomitsu High School, attending class 1-3, and is known for her energetic, athletic, and markedly tomboyish personality. Her appearance reflects her active lifestyle; she has short, light brown or platinum blonde hair, purple eyes, and a slender yet fit build. She consistently refers to herself using the pronoun "boku," a typically boyish form of self-address, and her personal motto embodies a stoic belief that failure is merely proof of insufficient effort.

At her core, Iku is defined by her extreme passion for baseball and her unique masochistic tendencies. She is the last remaining member of her school's girls' baseball club, as the other eight members left to study the sport in the United States, leaving the club on the verge of disbandment. This situation drives her to tirelessly recruit new members. Her training regimen is famously brutal; she has performed over one hundred thousand practice swings and pushes her body to its breaking point, finding genuine pleasure in the resulting physical pain and exhaustion. This manifests in her habit of exclaiming "That's so rough…♥" with heart-shaped eyes whenever she experiences hardship. Her masochism extends beyond sports, as she also enjoys terrifying horror movies and punishingly difficult video games. A notable and recurring gag is her love of being hit on the rear with a baseball bat, a practice she calls "butt bat," and she has a strange ability to produce a bat from seemingly nowhere to request this.

Despite her love of pain, Iku is not without her boundaries and insecurities. She prefers hardship that is tied to a meaningful challenge or training, and she does not derive pleasure from genuinely cruel or emotionally scarring situations. She also harbors a sense of insecurity regarding her boyish appearance, often feeling that feminine clothing does not suit her and becoming flustered when faced with direct romantic or sexual teasing. Her backstory reveals that her masochism developed from her childhood experiences with baseball. Encouraged by her older brother to persevere through the pain of practice, she came to associate suffering with improvement and success. However, she also carries a specific trauma from her past: a home run she hit accidentally injured a spectator. This memory caused her to freeze up and be unable to swing the bat properly for a time. With Rentarō's support and the encouragement of her fellow girlfriends, she was able to overcome this fear and reclaim her full power at the plate.

Within the Rentarō Family, Iku plays the role of a cheerful and energetic mood-maker who gets along easily with others. Her resilience and high tolerance for pain make her a sort of "tank" for the group, often volunteering to face dangerous or harsh situations first. She has notable dynamics with several other girlfriends. She shares a particularly symbiotic relationship with Rin Baio, who loves to inflict violence, as the two enthusiastically enable each other's preferences. In contrast, she experiences despair when the doctor girlfriend, Moegi Momiji, uses her massage skills to erase all the muscle soreness Iku cherishes. Her incredible physical endurance and ability to bounce back from punishment have led other characters to describe her as "undead" or a physical monster, often grouping her with the strong-willed Karane Inda as one of the most physically formidable members of the family.