Dudley’s best friend (PS2).
Piers Polkiss helped Dudley beating up others. He attended Smeltings with Dudley (PS3) and as present when Harry let escape the snake at London's Zoo.
Other canon notes and references
Piers calls Dudley Big D (OP1).
As Piers was the one to call to attention what the snake was doing when Harry spoke to it, Piers must have heard Harry speak parseltongue.