Reese

"She really is just a scared kid who unwittingly taught her toys how to become a weapon of destruction."

- Daniel Jackson

Reese was an android with a child's consciousness. She created the Replicators for her to have as toys, but she lost control over them and her whole planet was destroyed. She was taken to Earth by SG-1 before they realised what threat she posed. She made a good, though somewhat jumpy, connection with Daniel Jackson whom she started to regard as a friend. She was killed by Jack O'Neill when the base was attacked by the Replicators, but before she died she shut the newly created Replicators off.