Horikita was born on October 6, 1988 in Kiyose, Tokyo, Japan. She is the eldest of three daughters. Considered a tomboy in her childhood, Horikita enjoyed playing basketball and baseball. She was the vice-captain of their basketball club in junior high. Despite her boyishness, Horikita looked up to her mother. This was revealed when she appeared in an episode of KAT-TUN's now-defunct variety show Cartoon KAT-TUN where she mentioned that she liked imitating her mother when she was younger.