Yuichirou Yamaguchi is a father of Tsutomu Yamaguchi and husband of Kimiko Yamaguchi. He also is friends with Mikako's parents, Hiroaki Tokumori and Noriji, they all always arrange celebrations (and drink a lot) together.
Yuichirou usually wears ordinary, age appropriate clothes. Mostly he is shown in a suit or pajama.
Personality and traits
He is easygoing, nice, friendly, humble, peaceful and cheerful man. Yuichirou likes to play Famicom (video game console similar to Nintendo) with Tsutomu.
Behind the scenes 
Yuichirou Yamaguchi is ancillary character.
He hasn't been named until volume 2, chapter 6, corner talk on page 21. Yazawa Ai asked readers to suggest names of Tsutomu's parents in corner talks of previous chapters. She wrote that readers sent her mountains of postcards with suggestions, from which she picked their names.
- The Yamaguchi Family's Tsutomu-kun (First appearance)
- Nice Body-ko (Only in a corner talk, where he first time was named)
- The Same Birthday
- Studying Abroad in London
- To Love Someone: Part 2
- Being Adult