She was originally the strongest Sailor Soldier in the galaxy, but in fighting Chaos, she was corrupted. Her goal then became to gather all the "true Star Seeds" from all sailor soldiers to rule the galaxy.
When fighting Chaos (in flashbacks), she sent out her own Star Seed, her 'light of hope', which took shape as Chibi Chibi. Sailor Galaxia then attempted to seal Chaos within herself. As time passed, Chaos took over her body, and the last part of Sailor Galaxia herself was sealed away within her own body.
She killed Mamoru on his way to college in America when she captured his plane. Her powers were too strong for all the Sailor Soldiers, expect Sailor Moon, who rid her of Chaos.