Gollum

Gollum was the Hobbit once named Sméagol whose mind and body were corrupted by the One Ring, turning him into a creature capable of both good and evil deeds.

My precious
While fishing with his cousin Déagol, they found the One Ring in the river Anduin. Both came to desire it for themselves, thus they began fighting for it and ultimately Déagol was killed and Sméagol got his precious. He was banished by the other hobbits and slowly but surely his mind and body were corrupted by his prize's power. The Ring drove him mad, thus he became lonely which caused him to want someone with whom to speak, resulting in a personality disorder: Sméagol (what remained of the good in him) and Gollum (what the Ring made him into, a murderer and thief).

We hates it forever
During the Quest for the Lonely Mountain, Bilbo Baggins stumbled upon Gollum and took the Ring. The creature realized this, but before he could take back his precious, the hobbit became invisible and escaped. Gollum swore to once recover his stolen property and that he would hate Baggins forever.

Torture
He was captured by the Shadow's servants and brought to the Tower of Barad-dûr where he was tortured and interrogated by Orcs about the location of the One Ring. After he revealed that Baggins from the Shire stole it from him, he was freed, with the Dark Lord Sauron promising him the precious in exchange for his services. Gollum agreed and made his way to the mines of Moria.

Finding Frodo Baggins
He followed the Fellowship of the Ring to Amon Hen and later tracked the hobbits Frodo Baggins and Samwise Gamgee. The former was aware of his presence and managed to capture Gollum. He was tempted to kill him, but chose not out of pity. They made a deal with the creature in which it would be freed if it showed them the way to Mordor. Gollum agreed because it was the first time someone showed him compassion and guided them on the road to the Black Land. For a short time, Sméagol resurfaced. However, Gollum returned and he made plans to feed the hobbits to the spider Shelob and then get the Ring for himself.

Trivia

 * In Ralph Bakshi's The Lord of the Rings, Gollum is revealed much later to have been tracking the Fellowship of the Ring but how he did this and also his escape from Mordor is not told much of. In the books, he was indeed captured by the forces of the Dark Lord and tortured until he told them of Baggins and the Shire. Sauron promised to give him the One Ring in exchange for his allegiance and once the deal was made, he released Gollum and sent him to Moria to wait for their quarry. He is briefly seen a few times by both Frodo and Gandalf in the mines.