The Raston Warrior Robot was a robot created by the Janitor to serve as a guardian of the Halo Installations. It can be assumed that he was a prototype of his kind, but he went rogue and ransacked the Raston Science Facility in a short Skirmish with the security forces there, then fled.
Ever since then he has scoured the Galaxy, fleeing and killing wherever he goes. The Robot itself is incredibly powerful, with abilites like teleportation, shielding, self-repair nodes, an unlimited power supply, neural erasers that destroy living tissue, lasers and disk launchers.
The Robot has made it into the legends of many different cultures, and since it is almost indestructible, it has slain many warriors. Eye-witness accounts even claim it can perform feats of unimaginable ferocity, like wiping out an entire squadron of Cybermen without even so much as a scratch.
The Robot has not officially joined the League but it has never attacked League delegates or members before, and rumour has it that it feels some affiliation with the League since the League despises Forerunners. The robot also has close association with the Geth. Because of this, it has been invited to join, and maybe one day it will.