Shinkai Tsuki is the main protagonist of the Matchmaker story in The 25th Ward: The Silver Case. His is a member of the Regional Adjustment Bureau and his partnered with Yotaro Osata.
Shinkai Tsuki is a harsh, level headed member of the Adjustment Bureau, hired by the chief of the Bureau, Kiryu. He is shown to be very critical of Osata, and is usually the go to man to clean things up for both the Bureau and Osata's mistakes.
Though he is shown to be initially cruel to Osata, he is shown throughout the course of the story to have a soft spot for him and makes strides to keep Osata out of harms way. He is also aware of Osata's forgotten past and was given the task of keeping him from "awakening".
When it is revealed that he was once a member of the Okiai Syndicate, it is shown that he was similar to Osata, as he was a nervous greenhorn and was very obedient toward his peers.
Shinkai Tsuki was originally a regular thug until he was picked up the Ishiki, a member of the Okiai Syndicate, the last surviving yakuza in the 24th ward. During this time he is shown to be at the lowest of the totem pole and is Ishiki's assistant. One day Shigino and Ishiki order him to bring a handbag that is in a box to a storage unit by the bay owned by the syndicate. Once he gets there he discovers that not only was he set up to be a part of a drug deal centered around the bag actually containing dope, but that both him and the buyers were sold out by the police, getting him placed in prison for a few years.
After getting out of jail, he was sought out and recruited by Daigo Natsume to be apart of the Republic. There he trained alongside Kenichi Sakamoto, Haruhiko Inomata, and even Akira (The Silver Case), who he refers to as being someone "kinda like a chinchilla" as he cannot remember his name. It is then revealed that he was badly injured while training under the republic, and that while he was hospilized, the events of the Decoyman chapter in The Silver Case took place. After the failed attempt to capture Kamui Uehara by the Republic, it was disbanded, leaving Tsuki jobless until he was found by Kiryu and recruited into the Regional Adjustment Bureau.