The Shopkeeper

The Shopkeeper (店長) is the cook and owner of Itan's Ramen Darou. He speaks extremely quietly, and is also bad at communicating. Due to also being bad at communicating, he shares an empathy between Shouko and much like her family, can have a conversation without speaking.

He (and, by extension, Ramen Darou) has five Golden Rules. They are as follows:
1. Thou shalt not speak.
2. Thou shalt immediately buy your ticket and promptly take your seat.
3. Thou shalt uphold manners, respect the shopkeeper and wait with peace of mind.
4. Thou shalt not touch your cell phone.
5. Thou shalt return your bowl once you finish eating and immediately give up your seat.

Komi, having already been shown to share Shopkeepers lack of communication skills, is also adept at following his rules, and is shown to be a model customer.

He debuts in episode 5 of the anime, and chapter 65 of the manga.

(Source: Komi-san Wiki, edited)