Kei Yuuki is the childhood friend of protagonist Tadayasu Souemon Sawaki and, like him, a freshman.
His parents run a sake brewery (making them customers for the mold starter from Sawaki's parents). Kei disappears from the story soon after he and Sawaki enter college. He comes back dressed in black Gothic Lolita clothing and claims he has stronger feelings for Sawaki. Through out the series Kei's concern and devotion to Sawaki is seen on multiple occasions. Also worth noting is two Kei lookalikes Yū Kaneshiro a girl from Okinawa and second girl called Marie a Gothic Lolita from France whom Kei would later develop a friendship with.
Kei's personality is at first demure and shy when he first sees Sawaki again but after being found out he reverts back to his usual way of acting albeit with some quirks. Later in the manga Kei starts to focus more on sake brewing and helping Professor Itsuki with his projects. He also works at Hiyoshi Liquor store helping rearrange the store front *with somewhat comedic effect* and also serves as the bartender at times in the hidden bar in the back of the shop.
Reason For Cross-Dressing
Kei's crossdressing is met with confusion by Sawaki partly because Kei had always disliked people calling him "girly" or commenting on his feminine appearance in his youth. Kei's reasons are elaborated that he wanted explore as many different and extreme things as possible while at university and the anime alludes further to him wanting to see if there's some behind even the things he "hated". Later in the manga Kei shows up in his old male clothes to say goodbye to Kaneshiro much to the shock and confusion of everyone around him, Professor Itsuki later comments that most people wouldn't understand or accept the answer for why Kei's dresses like a girl. Kei is seen a second time non-crossdressed after meeting Marie for the first time in America,when pressed about it during their trip to New Orleans both Marie and Kei air their personal feelings about one another and immediately change back into their usual Gothic Lolita attire.
-Infamous Lolita fashion brand Baby the Stars Shine Bright (BTSSB) has made two dresses based on Yuuki Kei one inspired by the live action TV show and a much more character accurate piece to coincide with Season 2 of the anime.