He is one of the postmen working on planet Aqua. He was the first person Akari Mizunashi met when she came to the planet, and also the one who brought her then sleeping self over to Aria company when he learned she would start working there.

Spoiler

Akari eventually learns about his real name after she becomes a prima and Mr. Postman hires her, thus becoming her first paying customer.

(Sources: Aria the Animation, Natural, Origination episode observations)