Yui Yuigahama is a student in Class 2F of Sōbu High School and the first official customer of the Service Club. She later becomes the third member of the Service Club.

Yui's appearance varies from book to book. When first introduced, she is described to have loose, shoulder length chestnut-brown hair that would sway when she walked. She is also mentioned to not follow the dress code, wearing her skirt short, the top three buttons of her shirt undone, and a heart charm on a gleaming necklace.