Mizuhara Riku is a 1st year from Nekochiya. He lives in the same pension house as Ten, Chiaki, and others. 

Riku has a very outgoing and warm personality, completely the opposite to Chiaki. He is often mistaken as a player since he goes out of his way to praise and make happy to all girls he encounters, often calling them 'cute'. 

~!With the introduction of Mizuhara Rei, we discover that Riku has a mutual troubeling relationship with him. It is foreshadowed and implied in future chapters how Riku was living with Rei, but due to unknown circumstances moved out to the pension house. Riku does not want to return to Rei's house so much that he often spends all of his school and season breaks at the pension house.

__Information not revealed in translated chapters:__ Later on we learn that Riku was adopted into the Mizuhara family with his biological mother being Ayame Fujiyoshi.!~

__Relatives:__
~!Ayame Fujiyoshi (Mother)
Hotaru Fujiyoshi (Older Sister)
Mahoro Mizuhara (Adoptive Mother; Deceased)
Arata (Adoptive Father; Deceased)
Yukiji Mizuhara (Adoptive Grandfather)
Rei Mizuhara (Adoptive Brother)!~

__Relationships:__ (Major spoiler)
~!Ten Serizawa (Girlfriend)!~

Ten Serizawa has a two-hour commute through the mountains to high school every day, so she can’t spend much time hanging out with her friends in the afternoon. She decides to move into the local boardinghouse, where one of her friends and three other boys are living. Ten’s friends consider her to be as oblivious as a rock when it comes to noticing boys and falling in love, but will she be able to keep her calm and steady heart in her new living situation?
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(Source: Viz Media)

Riku Mizuhara