Demelza Robins was a Hogwarts student in Gryffindor House during the mid-1990s. She played Chaser for the Gryffindor Quidditch Team that won the Quidditch Cup in 1996-1997 (HBP24).
She was added to the team after a good tryout and worked well with Chasers Katie Bell and Ginny Weasley (HBP11).
Demelza brought Harry an unwelcome message about his detention with Professor Snape (HBP11).
She was later was accidentally punched in the mouth by Ron Weasley during one Quidditch practice session, but generally performed well in these sessions and on the Quidditch pitch (HBP14, HBP19).
Demelza was "particularly good at dodging Bludgers" (HBP11).
Demelza is a Cornish place name meaning "fort of Maeldaf" (Behind the Name). It was popularised as a name for girls in Cornwall by author Winston Graham, who named his main female character Demelza Carne in the "Poldark" book series after the hamlet of Demelza in Parish of St Wenn (Discover Poldark: The real places behind the names).
Robins is an English and French surname derived from a diminutive or nickname of the name Robert, which itself comes from the Germanic name Hrodebert, meaning "fame-bright" (Internet Surname Database).
As Demelza only appears in the one book, it is unknown whether she is either the same age as or younger than Harry Potter and the other team members. She may not have wanted to try for a place on the Gryffindor Quidditch Team until Chaser-Captain Angelina Johnson and Chaser Alicia Spinnet had left Hogwarts.
Lexicon list of Chasers
Demelza House is the favourite charity of actor Daniel Radcliffe.
From the Web
Pottermore feature: All about Gryffindor
Harry Potter Wiki: Demelza Robins