Dennis Creevey was the younger brother brother of Colin Creevey, and a Gryffindor student from 1994 (Harry‘s fourth year) to 2001. Muggle-born and with “mousy” brown hair like Colin, Dennis was the smallest kid in his year when he arrived, and thus almost certainly the smallest kid in Hogwarts; he needed to stand on his seat in the Great Hall to be able to see the Goblet of Fire properly, and still only just came up to everyone else’s eye-level (GF12, GF16). However, Dennis is another example of the Weasley twins’ saying that “size is no guarantee of power” (OP6), in that he seems to know no fear.
On his first-year journey across the lake in a storm, Dennis fell in and was rescued by the giant squid, which seemed to him to be all good fun (GF12). Dennis and Colin tried to modify a stack of magical Support Cedric Diggory! / Potter Stinks badges to say Support Harry Potter! instead, but all they could manage was to get them stuck saying “Potter Really Stinks” (GF19). Dennis, along with his brother, joined Dumbledore’s Army in October 1995, and learned to do defensive and some offensive magic from Harry (OP16).
Dennis was in his second year during the 1995 – 1996 school year, but somehow managed to get into Hogsmeade and attend the initial meeting of Dumbledore’s Army at the Hog’s Head. He shouldn’t have been allowed into Hogsmeade until his third year, unless they changed the rules at some point. Or possibly the Creevey parents, who must be much-tried people, felt they’d save themselves a lot of stress if they just did both brothers’ permission forms at once.
Father - Muggle Milk Man
Brother - Colin Creevey
Learned some offensive/defensive magic from being in Dumbledore's Army.
Other canon notes and references
"Creeve" ="to burst," suggesting the Creevey brothers' excitability.