Bastian was a male mouse and a member of the Mouse Guard. A Patrol Guard since 1138, Bastian's specialty was hunting predators. Unlike his fellow Guardmice who tended to avoid them unless a conflict was necessary, this orange-cloaked mouse felt that killing beasts known to eat mice and prevent their settling in the Mouse Territories made life better for all mouse-kind. His determination earned him a few battle scars, but none were serious enough to deter him.
During the winter of 1152, Bastian - alongside Aubrey, Delvin and Sienna - was sent to the settlements of Elmwood, Whitepine, Dawnrock, Thistledown and Wildseed by Gwendolyn to gather supplies. He was also present at Celanawe's funeral later that season. The following spring, Bastian and Delvin were sent by Gwendolyn to hunt down a snapping turtle attacking the settlement of Grasslake.He peppered the turtle full of arrows so Delvin could slay it by slashing it across the throat.
- Bastian's name is most likely a tribute to David Petersen's real-life friend Jeremy Bastian, writer and illustrator of the story The Battle of the Hawk's Mouse and the Fox's Mouse in Legends of the Guard Volume 1 and Archaia Entertainment's Cursed Pirate Girl.