Bregalad is the Elvish name, which translates to "Quickbeam"1, for a hasty Ent considered rash by his peers. He received the nickname for saying "yes" to an elder Ent before the elder had finished his question. Another sign of hastiness was that he drank so quickly that he was leaving before others had finished wetting their beards. During the Entmoot which led to the march upon Isengard, Quickbeam entertained Merry and Pippin, having made up his mind long before his brethren. He loved to laugh, sing and sway before the rowan-trees and he even sang to the Hobbits.
Physically, the hobbits noticed that Quickbeam was tall and seemed to be one of the younger Ents. His arms and legs had smooth, shining skin. His lips were ruddy and his hair grey-green.
Since the entmoot took a long time, Quickbeam took the hobbits to his nearby ent-house. It was a mossy stone set upon turves under a green bank. (A turf is sod, which here is a thick mat of dirt full of grass and roots. Merriam Webster) Rowan trees encircled it. As in all ent-houses, there was water: here a spring bubbling out of the bank.
Quickbeam explained to the hobbits that he belonged to Skinbark's people, and the rowan-trees they planted to attempt to please the ent-wives had been cut down by Orc-axes. Oddly, just before that, the birds that had lived in those trees had changed temperament, greedily tearing at the trees and throwing down the fruit uneaten. This disaster was why his mind was already made up to fight.1
Quickbeam went with the other Ents to fight at Orthanc, accompanied by huorns, which are Ents that have become "treeish".3 The battle is told in detail by Merry and Pippin to the rest of the Fellowship. The Ents waited, watching Saruman's forces march out towards Rohan. The huorns, hating the Orcs, followed after them. Most of the Ents, not all, stayed at Orthanc, wiping out or terrifying out the remaining Men and Orcs, ripping into the fortifications. At night under starlight, Quickbeam spotted Saruman and lost his gentleness, chasing him, coming within a few steps of catching and strangling him.
Saruman, once inside his tower, wasted no time in starting his machinery, sending up masses of fire. Only the tower of Orthanc's special material saved him from the Ents then. Treebeard brought his people away from there and they set to work that day, both ents and the remaining huorns. Watchers remained hidden around the tower, and it may be that Quickbeam was one. Gandalf came up briefly for a discussion with Treebeard and left. Most of the rest of the Huorns followed Gandalf not long after. About midnight, the Ents released all the waters around that they had collected and washed it all into Isengard, drowning the whole place. By morning, the Huorns returned to Orthanc, and so did Grima Wormtongue who was forced to go into Orthanc, likely watched by door-guard Quickbeam.
Saruman was imprisoned in his tower and the Ents intended to watch over him with the returning trees, calling it the Watchwood. It is very likely that Quickbeam was among them, continuing as a guardian set over Orthanc and Isengard.3
At Orthanc after the battle, Quickbeam acted as one of the guards over Saruman, with the help of a lot of water. Legolas said he appeared to be a tall, grey door-tree.2