Ulmo is a member of the Valar in the mythology of Middle-earth, as explained by The Silmarillion by J.R.R. Tolkien. He can be compared to Poseidon from Greek lore, and is the master of the Sea. He is single (most of the Valar are paired up) and roams the waves, homeless and wandering, refusing to be bound to a single location.
Among the Valar, he is most concerned with the doings of the Children of Ilúvatar, or elves and men. He only comes to Valinor on occasion, and usually to supplicate on their behalf. His voice is like the sea, coming from the rumbling deep, and he inspires great awe.
It was Ulmo who told Finrod Felagund where to build Nargothrond. He watched over a great many of the events that occured in the Silmarillion, and was instrumental in the redemption of the Noldor from the doom of Mandos.