Grawp
Age: Somewhat younger than Hagrid
Distinguishing Characteristics: Sixteen feet tall. Disproportionately large and perfectly round head covered in curly, bracken-colored hair.
Heritage: Giant
Family: Father unknown. Mother is the giantess Fridwulfa. A half-brother, Rubeus Hagrid.
First Mention: First mentioned explicitly in chapter thirty of Order of the Phoenix.
General: Born to the giantess Fridwulfa following the desertion of her partner and son, Grawp was raised in a community of giants in their mountain dwelling. However, at "only" sixteen feet tall, he was terrorized by the other giants due to his relatively small size.
Grawp was discovered by the half-giant (and his half-brother) Rubeus Hagrid, on a mission in which Hagrid and Olympe Maxime were sent to make negotiations with the giant community by Albus Dumbledore. These negotiations were not entirely successful, but Hagrid, upon learning of his ties with Grawp, decided not to allow him to remain where he was, instead bringing Grawp back to Hogwarts. Grawp lived in the Forbidden Forest, ripping up trees and terrorizing the local wildlife, while Hagrid tried to teach him how to be "civilized," all the while keeping his presence a secret.
With the prejudiced and tyrannical regime of the temporary headmistress Dolores Umbridge closing in on him, Hagrid finally decided to share the secret of Grawp's existence with Harry Potter and Hermione Granger, asking them to "look after" Grawp should Hagrid be forced to vacate the school. Luckily the two students were spared this burden, as Hagrid was not driven from Hogwarts until the end of the school year.
Shortly afterward, when Harry and Hermione were coerced by Dolores Umbridge into showing her the location of a nonexistent "weapon", they were waylaid by the centaurs of the Forbidden Forest. Umbridge's prejudice against part-humans led the centaurs to attack her--but they then turned on Harry and Hermione. It was at this stage that Grawp, desperately looking for Hagrid, came to the rescue of the two, scaring the centaurs away and allowing Harry and Hermione to escape.
Upon Hagrid's return to Hogwarts, he stated that Grawp's behavior was "loads better". Indeed, Harry and his friends did not see any sign of Grawp during their sixth year, except when he attended Dumbledore's funeral. (He was very well-behaved.)