Ulmo (Quenya, pronounced [ˈulmo]) was a Vala, also known as King of the Sea, Lord of Waters, and Dweller of the Deep. One of the mightiest of the Ainur, he was second in majesty of the Aratar, after Manwë and Varda.
At several points Ulmo was notable for setting himself apart from his brethren, and also for being the friendliest to the Children of Eru. Ulmo is the Lord of Waters and King of the Sea.He is the Ainu most deeply instructed in music, and it is said that in water the Eldar hear the echo of the Music of the Ainur.He lives in the deeps under Ambar who alone has seen where he devises his music and whence he governs all waters, bays and rivers.
From the beginning Manwë has been his closest friend and ally and they come together when the vapors of the water become clouds high in the air, or mists and dew. The two most faithfully served the purpose of Eru. Ulmo was unique among the Valar in several ways; he was alone, taking no spouse among the Valier; he didn’t dwell in Valinor but in Ekkaia from the beginning, preferring the deeps of the seas around and below Ambar, and the rivers. He seldom appeared to the councils of his brethren, save in matters of great importance.
Likewise, he seldomly wore a fana but he came to the shores unseen or entered firths of the sea and rivers and it was said his spirit was in the very veins of the world. He spoke through the sound of water, and made music with his horns the Ulumúri. But whenever he appeared, he was dreadful and terrible “as a mounting wave that strides to the land”; he wore a dark foam-crested helm and a shimmering silver-green mail, and his voice was deep like the ocean.
Ulmo cared about Arda and the Children of Eru, and through the veins of the world he kept in touch with them and saw every grief and need, and thus knew more of the goings on with them than even Manwë. Even while the Valar were secluded in Valinor or when the Children were under the wrath of his brethren, Ulmo, alone of the Valar, was the one who never forsook them.
Source: The History of Middle Earth, Tolkien Gateway and Wikipedia.
Artwork: Ulmo And Tuor art by DanielPilla on DeviantArt.