He is the son of Moritz Brunnen, a powerful man who has political power, and so boasts of his family name.
Typically is seen around town with Lubart Valentz, often bullying Tao and bickering his group of friends for being a burden on the island. This wasn't the case long ago as he used to be friends with Ryza, Lent, and Tao but after an incident in the past they began fighting.
When Bos was a child, he was exploring a flooded area of the island with Ryza, Lent and Tao. When she fell into the water, he ran to find Agatha while Lent and Tao were trying to pull her to safety. He knew his father had sent Agatha to watch him and she would be able to help. Tao didn't see it that way and the others agreed with him which caused them to drift away from him and start to criticize one another until in turns to a constant reprimanding when they see each other.
When Ryza and his group are growing and begun being acknowledged by the residents of the island, he got self-conscious about this and wanted to prove he is better than them. As Ryza started making a name for herself as an alchemist, he wanted to uphold the honor of his family name. He would go out on several excursions to the mainland, only to end up being rescued by Ryza and her friends.
Eventually he runs away with Lubart only to get separated and be chased by Philuscha into the Underworld. There he meets Kilo Shiness who protects him until Ryza arrives. While there he learns his family's water source is stealing water from the Underworld and makes amends with Ryza.
When he returns to the island he reveals the source of water is a glowing orb and intends to destroy it, but is stopped by Lila since the people on the island still depended on it. While Ryza was looking for a more permanent solution, he occasionally spent time with Lent for training combat and Tao learning more about the island.
He stays behind during the battle with the Ravaging Queen but returns to the underworld to destroy the orb after Ryza starts a desalination machine on the island.
As of the end of Atelier Ryza he intends to study in the capital with Tao.
- His name in Japanese "Bosu" is a pun on the term "Bozu" (坊主), an impolite form to address young boys.