The Guardian is an entity that spawns on the upper right corner of the main island. She wields a “GuardianSword” which deals 25 damage. She is the only friendly entity in the game.
He uses the old sword model (linked sword model).
His sword has 10B durability
He used to be spawned by the Guardian spawner.
Yes, the guardian is female. 
The Devs jokingly said that all entities are female, and if not then the guardian is female, but they also make them bald and male. (I got this info from wiki)