Born Marina Hara on the sixth of October 1988 in Kiyose, Tokyo, Japan, Horikita is the eldest of three girls. She was boyish during most of her childhood. For instance, she enjoyed playing basketball and baseball with her friends and preferred playing Dragon Ball Z with the boys than playing dolls with the girls in middle school.
Despite her boyishness, Horikita looked up to her mother. This was revealed when she appeared on KAT-TUN's variety show, Cartoon KAT-TUN, where she mentioned that she loved imitating her mother when she was younger.