This Uruk-Hai creature is most powerful of the army of Saroumane. To result hybrid from a crossing between an Orc and a Goblin, Uruk-Hai is strong and cruel. The white hand painted on its face is the symbol of Saroumane. (it does not appear in the novel of Tolkien)
It is him which kills Boromir on the windy Mount, it is killed then.
He is the chief of Uruk-Hai. After the death of Lurtz, it took the command of the troops charged to capture the Hobbits and to bring back them to Sarouman.
This Orc of Mordor took part in the attack against the Community of the Ring during which Boromir died and where Merry and Pippin were captured. Its greed and its silly thing make it possible nevertheless the two Hobbits to flee before it is not killed by the Men of Rohan.
Good it does not agree a captain of Mordor, but its role is of primary importance in the story!
Balrogs are demons of old times which have almost all perished. But one of them survived in the mines of Khazad-Dum. Accidentally released by the Dwarves, it all exterminated them before being killed by Gandaf.