Guosim was President of the Guerilla Union of Shrews in Mossflower (herself being named after the group), a tribe of shrews that lived a gypsy-like existence in Mossflower Woods that feared predators. Guosim was dearly loved by Log-a-Log.
When Matthias entered union 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. She later joined Matthias in his quest, for if he killed Asmodeus, Captain Snow would lose the bet (and never eat mice or shrews again), and Mossflower would be safe for the shrews to live in once again.
After tracking the sword to the home of Asmodeus in the Quarry, Guosim and Log-a-Log escorted Matthias into the quarry. The trio initially spent the better part of a day scouring the quarry-pit's edges for possible openings, but the tunnel network-lair of Asmodeus 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 Guosim sitting against a cave wall motionless. When Matthias got closer, she slumped over, and he found Guosim to be dead, having been bitten and killed by Asmodeus and dragged to the cave.
Guosim was the last victim of Asmodeus, as the adder itself was slain by Matthias.