The Other was to be a mysterious and powerful figure from Gallifrey's past who helped form Time Lord society, according to whispered rumors about the ultimate story goal of The Cartmel Masterplan.
Rassilon (Assilon? Bassillon - ah ha!) invented Time Lord society and Omega invented Time Lord technology while The Other invented being a vague ambiguous cunt. He's totally not the Doctor, you guys.
This is much the same information found on Cartmel's page, so why not go read about it there? The Other really didn't need it's own fucking separate page, this isn't Tardis Dildo Corps.
Although he is pretty spooky. Is he a Morbius Doctor? Is he Jesus Christ? Is he Dr. Who? Or is he just too ostentatiously obscure for his own good? DEEPEST LORE.