Items in his shop were all second-hand and date back to 50-70's. They are tasteful and cheap. He was buying them directly from foreign countries.
When Nana and Hachi were shopping at his shop, Hachiko got to know that his shop assistant suddenly left and suggested to hire her. But Seiichi wasn't very successful at his business, probably because he is too honest person, he didn't make almost any profit. Eventually, he had to close the store due to poor business and moved back home.
He married his childhood friend Harue and sent postcard with their wedding photo Hachiko.
Personality and traits
Mr. Mizukoshi seems to be a very nice, friendly, supportive, easygoing, and likable boss.
Behind the scences
*In manga his name spelled Seiichi, but in anime's info it spelled Keiichi. Kanji of his name 誠一 can be read as Seiichi or Kouichi, but not Keiichi.