Rein is the second protagonist of the series and the other twin princess of Sunny Kingdom. She may look identical to her sister, Fine, but her eyes are sea-green and her hair is blue and much longer than Fine's. Rein is also referred to as one of "the most unprincess-like princesses in the history of Fushigi-boshi", but unlike Fine, she is likes to dress up and wear elaborate decorations. Rein gets crazy when it comes to romance and always end Bright's name with "-sama". She's adventurous, but as friendly, eccentric and caring as Fine. Rein often thinks positive ways to fix things, and acts more feminine than Fine. Her name was originally spelled with an "a" in English, but the director, Junichi Sato, doesn't want her name to be related to the weather "rain".

Note: In the manga (and some of the episodes in the anime), Rein had some affection for Shade. Although she was with Shade in some of the episodes in the anime, she is much more in love with Bright.