The First One is a mysterious character that frequently appears in the SU fan series Legends, to which she is a big part and forms the general arc of. She is described to be impossibly old, older than the Diamonds themselves. Her identity is a mystery. She acted as the supreme ruler of the Gem Republic during Era 0.
Her power and influence has been described as 'beyond comprehension'. She was the inspiration for T'mn, the Era-one mythological gem goddess of creation. By then many had forgotten who she was or that she even existed. The only ones who knew about her believed she was a myth, and learned about it in schools for the elite.