Kei Yuuki

Kei Yuuki is Tadayasu Sawaki's childhood friend and a freshman at the Agricultural University in Tokyo. His family owns and runs a sake brewery.

Moyashimon
Kei Yuuki first appears in Episode 1 with Tadayasu Sawaki in search of Keizō Itsuki, whom the pair encounter shortly after digging up the corpse of a seal. From then on, they are found with Sawaki at nearly all times, and with him meets two new friends named Misato and Kawahama.

Sometime during the first half of Season 1, Kei Yuuki finds himself reflecting on college and his passions. These thoughts culminate with his departure from the show in Episode 6 following the guidance of Professor Itsuki. Kei does not warn any of his friends about his break and goes as far as to refuse to answer any and all calls from them.

Sawaki finally reencounters Kei at the end of Episode 9 just outside of Hiyoshi's Liquor; however, he does not recognize Kei due to him being adorned in gothic lolita attire. Sawaki spends the entirety of Episode 10 warring over his attraction for the mysterious lolita and just why they seem so familiar. In the end, Sawaki confronts the lolita only to be faced with the crushing reality that the girl was actually Kei.

Perhaps knowing that this would be one of his only chances to express his feelings toward Sawaki, Kei kisses Sawaki in the street. This kiss, much to the surprise of everyone, results in Sawaki losing his ability to see microbes, which would not be cured until the end of Episode 11.

At the end of Episode 11, Professor Itsuki announces that Kei will be returning in some capacity in the coming weeks.

Moyashimon Returns
At the start of Season 2, Kei Yuuki is still working at Hiyoshi's Liquor. Unlike the latter half of the previous season, Kei pays his friends frequent visits and often helps around with alcohol production. Despite this, Kei's role in Season 2 is minimal with his being absent throughout the bulk of the season.

During the Harvest Festival, Kei decides to aid his friends in securing enough funds to afford a flight to France in order to find Haruka Hasegawa. To do this, he joins forces with Oikawa -who dons a sexy, bunny costume, and poses in pictures for the university's resident pervert community. This initially proves successful, but later in the episode, the money that Kei and his friends procured is confiscated on the grounds that Misato and Kawahama illegally using school sap to make their Pulque.

At the end of the season, Kei joins Sawaki and the gang for drinks at Aya's bar. It is here that Kei learns that Sawaki met a French maiden of near identical physique to himself. Kei unconsciously reacts by knocking Sawaki to the floor, perhaps out of jealousy.

Trivia

 * Kei has a deep-rooted hatred for hiochi-bacteria. This is due to the bacteria having ruined his family's alcohol during an undisclosed year in the past, which ultimately resulted in the suicide of the brewer.