Guosim was the President of the Guerilla Union of Shrews in Mossflower, a tribe of shrews that lived a nomadic existence in Mossflower Woods that feared predators. She was named after the group itself. Guosim was dearly loved by Log-a-Log.
When Matthias entered Guosim territory in his search for the Sword of Martin the Warrior, Guosim was initially snappish towards him, but Log-a-Log convinced the shrews to help the mouse. She later joined Matthias in his quest, for if he killed the adder Asmodeus, Captain Snow would lose his bet with Squire Julian Gingivere and never eat mice or shrews again, ensuring Mossflower would be safe for the shrews.
Guosim and Log-a-Log escorted Matthias into the Quarry, the residence of Asmodeus where they had tracked the sword. The trio initially spent the better part of a day scouring the edges for possible openings, but the interior tunnel network appeared to be sealed. Guosim momentarily sat down on a large rock and began telling the others of their possible next move, but then the rock, revealed to be a hatch, gave way and she fell into Asmodeus' lair.
Matthias and Log-a-Log later scaled the tunnel down, but upon arrival, couldn't find sight-nor-sound of Guosim. Just as Matthias located the cave where Asmodeus slept, Matthias noticed a motionless Guosim up against a cave wall. When Matthias investigated the scene, she slumped over, and he discovered she was dead, having been bitten and killed by Asmodeus, and then dragged to the cave.
Guosim was the last known victim of Asmodeus, as the adder was slain by Matthias.