The Boy skin is the default avatar along with some others that require nothing in order to play as, Just starting the game will unlock this character for the player
The avatar has a red shirt and a blue pair of trousers, it has a female alternative with the same clothing.
Before the multiplayer update that included new skins every player would spawn as a very basic looking male character, who also had a shirt (which would have different colours for guests on splitscreen).
The default text for the boy Avatar Skin when it is placed by a NPC block block is
"Hello. Good bye. I must be getting home."