Ken Doi is the man in charge of the Ramen Don Ramen shop. Several times through the story, you will encounter and have to assist Doi to complete your missions, specifically when the new ramen shop, Shadow Ramen appears. Ken Doi also appears to be able to resist the imprinting of the Red Skull Pins for a short amount of time. Ken Doi is also apparently the one who left the boxes with the items for Neku and his partners to find.
 Another Day
In Another Day, Ken Doi's shop is used an an official headquarters for Shooter, Neku, Shiki, Beat and Joshua after their pins are stolen by Uzuki, Kariya and Higashizawa. it is also revealed that Ken Doi was the creator of Tin Pin Slammer, although he left the business after his last pin design proved too expensive for the company.