Rubeus Hagrid, groundskeeper at Hogwarts School of Witchcraft and Wizardry, is a major character in the Harry Potter series. He's played by Robbie Coltrane in the films.
We're first introduced to Hagrid when he arrives at the Dursley's house in the night to deliver baby Harry to Albus Dumbledore following the death of Harry's parents at the hands of Voldemort. He's extraordinarily large, standing some eleven feet tall (8'6" in the films) and reportedly three times the width of a normal man (or sometimes five times). To complete the intimidating look, he has an immense tangled beard, bushy eyebrows, and shaggy hair, which nearly hide his sizable face.
Hagrid, though terrifying when angry (and when not), is actually exceptionally soft-hearted, especially with regard to helpless dangerous animals. He befriends numerous magical beasts, including his boarhound Fang, a monstrous three-headed dog (Fluffy), a fire-breathing baby dragon (Norbert), a giant spider (Aragog), and many other deadly beasts. This occasionally results in trouble for him and his friends, but he has complete blind spot for the dangers these creatures pose. He serves as both a bit of comedic relief, as well as providing the occasional plot advancement when he accidentally lets slip some secret detail of which he has knowledge and that Harry really needs to foil the current evil plot that everyone else is oblivious to or in denial of.
Spoilers for Harry Potter (all books) follow
Hagrid attended Hogwarts in the early 1940s, some fifty years prior to Harry's first year, and the same time that Tom Riddle (Lord Voldemort) was attending. He was, however, expelled for having opened the Chamber of Secrets, releasing a beast that killed a student. Harry later proved that Tom Riddle had been the responsible party, with his own fell beast, so Hagrid was exculpated, but seeing has he was in his sixties by that time, Hagrid did not resume classes and continues to be largely a non-magic user.
Hagrid's mother was a giant, and as a half-giant himself, he is somewhat resistant to the effects of magic. He kept his half-giant status secret, as there is a good deal of prejudice against giants in the wizarding world, but it was revealed by Rita Skeeter during Harry's fourth year, resulting in both an outpouring of support from alumni as well as calls for him to be sacked.
The following summer, on a diplomatic visit to the giants, he discovered that he had a half-brother, Grawp (a full-blooded giant, albeit a relatively small one), whom he brings back with him, and keeps secretly hidden in The Forbidden Forest around Hogwarts. Grawp proves both a challenge to care for, but also an asset when it comes to fighting the Death Eaters in the final Battle of Hogwarts.