He is the son of Kisaragi, the nephew of Hazuki, Minazuki and Uzuki, and the uncle of Harunobu, Hiroshi, Kuranosuke, Masami and Masaru. In the Manga he's the brother of Denshichi, Izō, Kazuki, Miu, Saburō, Shūsaku, an unnamed brother, another unnamed brother, and ten other brothers. In the GDW Anime he's the brother of Izō, Kichinosuke, Kogorō, Shinsaku, Shintarō and Shūsaku.
Shinsaku is a black-and-white Siberian Husky and Alaskan malamute mix. He has a mainly white face but black circles around his eyes, similar to a panda.
Shinsaku lives with his brothers and father in Mutsu.
He is first seen hunting a bear with his brothers. After hearing from Shigure about the invasion of Ōu by Hōgen and Genba, he and six of his brothers are sent to Ōu to aid them. He and his brothers fight Genba's minions who try to steal females and come in time to rescue Chiko in time with Shigure and Akame He and his brothers later fight Genba's platoon and in commando to slay Hōgen.
Shinsaku and all the Mutsu brothers are sent to Hokkaidō with the Ōu soldiers to help Hakurō and his pack who were getting invaded by the Russian Army dogs. Some of Shinsaku's brothers were killed in battle.
After the volcano explosion in Ōu, Shinsaku and his brothers found Weed's daughter Bellatrix, who was lost in the snow storm. While Izō looked after Bella, Shinsaku and the rest of his brothers tried to find something to help Bella. Their search was interrupted because a Kurohabaki Clan member attacked Izō and kidnapped Bella. Luckily, they manage to stop him, but a second dog appears and rekidnaps the young Akita mix. While four of their brothers pursued the second kidnapper, Shinsaku and his two brothers Saburō and Izō, killed the evil dog.
Later Kisaragi's sons made recognition with Sasuke, Shirozaru and his brothers Muu and Rara. Muu and Rara looked after Bellatrix, while Izō, Shirozaru and Sasuke scouted the area. Soon after, they get into a short fight with Masashige and his pack. During the fight, Izō lets loose a howl to cause an avalanche. Shinsaku and the others jump into the snow to avoid damage, however, Shinsaku was crushed by the snow along with Kazuki.
- In the GDW anime, there is another Shinsaku which is a different character. This character also appears in the manga, but his name is unknown there.
- He and his brothers are called Alaskan Malamutes several times in Ginga Densetsu Weed, despite the fact that their father was mentioned to be a Siberian Husky.