Short answers, pulled from the story.
Meriadoc Brandybuck, called Merry, is a hobbit and one of the closest friends of Frodo Baggins in J.R.R. Tolkien's The Lord of the Rings. He is a member of the Company of the Ring, serves as esquire to King Théoden of Rohan, and plays a decisive role in the Battle of the Pelennor Fields.
During the Battle of the Pelennor Fields, Merry stabbed the Lord of the Nazgul behind his knee while the Black Captain was engaged with Éowyn, causing him to stumble and enabling Éowyn to kill him. This fulfilled the ancient prophecy that the Lord of the Nazgul would not fall by the hand of man, because the killing blow was delivered by a hobbit and a woman.
Merry and Pippin were given an Ent-draught to drink by Treebeard, the leader of the Ents, after escaping into Fangorn Forest. The drink made both hobbits grow unnaturally tall for their kind, and they returned to the Shire as its tallest hobbits.
Holdwine is the name given to Merry by Éomer after the Battle of the Pelennor Fields, when Éomer made him a Knight of the Mark for his bravery. The name is based on Old English and means faithful friend.
Tolkien maintained the fictional conceit that Merry's true name in the invented language of Westron was Kalimac Brandagamba, with the nickname Kali meaning handsome and happy. Tolkien claimed to be transliterating this into the plausible English name Meriadoc, from which the nickname Merry, meaning happy, could be derived.
Dominic Monaghan portrayed Merry in Peter Jackson's 2001-2003 film trilogy, playing the character as a cheerful prankster full of fun and practical jokes. Earlier adaptations cast Simon Chandler in the 1978 Ralph Bakshi animated film and Casey Kasem in the 1980 Rankin/Bass television version.