From the Story
"Marcus Flint is a fifth-year Slytherin and captain of the Slytherin Quidditch team. He is large and thuggish-looking and some classmates consider him as looking half-troll."
Discovered in Book 2, Chapter 7, Mudbloods and Murmurs
"Marcus shows off the Slytherin team's new Nimbus Two Thousand and Ones to the Gryffindor team after he has hijacked their Quidditch practice. "
Discovered in Book 2, Chapter 10, The Rogue Bludger
"Marcus is seen yelling at Draco Malfoy after the Gryffindor versus Slytherin Quidditch match because he did not catch the Snitch when it was right on top of his head. "
Discovered in Book 3, Chapter 13, Gryffindor Versus Ravenclaw
"Flint is involved in the attempt to sabotage Harry at the match, dressing up as a Dementor along with Malfoy, Crabbe and Goyle. He is given detention when they are caught, and has fifty points taken from Slytherin."
Discovered in Book 3, Chapter 15, The Quidditch Final
Flint is the Slytherin Quidditch captain. He makes some changes to the Slytherin line up for the Quidditch final, choosing players for their size, instead of skill.