SPOILERS START HERE, READ NO FURTHER IF YOU DON'T WISH TO KNOW SIGNIFICANT THINGS THAT OCCUR IN THE FIFTH AND SIXTH BOOKS.
In the series of Harry Potter books, Grawp (introduced in the fifth book Harry Potter and the Order of the Phoenix) is half-giant Hagrid's half-brother.
At the end of the fourth book The Goblet of Fire, Hagrid and fellow female French half-giant Madame Maxime set out into the mountains of France to try and ally with the alienated giants of the world on behalf of the wizarding world. To cut a long story short, the mission was a failure and the giants were intercepted by Voldemort's servants the Death Eaters.
However while Hagrid was there, he found out that after leaving his father, Hagrid's giantess mother had gone into hiding in the mountains and had a child, Grawp, with another giant before her death. Not having the heart to leave behind his only known remaining relative, his father being dead also, Hagrid sneaks Grawp back to the grounds of Hogwarts and hides his brother in the Forbidden Forest. He attempts to then civilize the giant, incurring serious injuries which do not go unnoticed by his student friends Harry, Ron and Hermione. When Hagrid goes into hiding (for reasons I forget), Hermione and Harry are left to give him 'lessons' which they conveniently forget to do.
However after Harry and Hermione are cornered in the Forbidden Forest by centaurs, Grawp saves their lives.
He then doesn't appear again until the end of the sixth book, attending Dumbledore's funeral considerably more civil than when we first meet him.