Hikaru, the fourth of nineteen Amatsuka sisters, is a 15-year-old first-year high school student born on January 24. With blood type A and standing 160 cm tall, she maintains a tomboyish, cool demeanor even in awkward scenarios, though kindness or emotional vulnerability easily flusters her. During family outings like river rafting and beach volleyball, her athleticism surfaces—she once lost her bikini top to a current during a game but swiftly managed the aftermath. Her physical reliability shines when aiding siblings, such as throwing a rescue rope during a raft mishap, though this act sprained her ankle. Her dynamic with adopted brother Yōtarō blends teasing and sincerity. She joins a lotion prank against him but intervenes before escalation. Later, after her ankle injury, Yōtarō carries her, prompting her heartfelt admission that his joining their family feels like a "miracle"—culminating in an impulsive kiss. Uniquely, this kiss stemmed purely from her emotions, as she was unaware of a maternal suggestion encouraging affection toward him. Hikaru’s internal struggle surfaces through visible embarrassment after intimate moments like the kiss, contrasting her usual cool facade. She openly acknowledges her romantic feelings for Yōtarō, marking a pivotal point in her character.