She always refers to him by the english word "darling". She tends to revert to a form of baby talk, which is a signal she's extremely immature and shielded. She has an unrequited love for Kyosuke, and has been Madoka's best friend for several years since she and Yusaku Hino were the only ones who were never afraid of Madoka.
Her name refers to the brightness of the sun, and of a sparkling personality. Hikaru is two years younger than Ayukawa and Kyosuke, and has the same birthday as Kyosuke.