A Gryffindor is:

The emblem of Gryffindor is the lion and their house colours are red and gold.
The founder of Gryffindor House is Godric Gryffindor.
The Gryffindor ghost is the Sir Nicholas de Mimsy Porpington though most Hogwarts students call him Nearly Headless Nick.
Godric Gryffindor's relic is Gryffindor's sword.