A red-haired Muggle boy whose appearance Harry uses in his “Cousin Barny” role at Bill and Fleur’s wedding (DH8).
Fred Weasley steals hairs from a red-headed Muggle boy from the village of Ottery St. Catchpole to use in the Polyjuice Potion that Harry drinks to disguise himself at Bill and Fleur's wedding. Fred uses a Summoning Charm to get the hairs from the Muggle boy.
Cousin Barny is really a Muggle boy, though is disguised as part of the large Weasley family.
Other canon notes and references
Cousin Barny (really Harry Potter) is also referred to as "Barry." (DH8)
Even though he is disguised, Luna Lovegood recognizes Harry immediately by his "expression." (DH8)
When Auntie Muriel meets Cousin Barny, she exclaims, “Another Weasley? You breed like gnomes." (DH8)
Harry, disguised as Cousin Barny, meets Viktor Krum as he questions Harry about Xenophilius Lovegood and his Deathly Hallows necklace. (DH8)