In A Midsummer Night's Dream, Puck is a fairy character. He is introduced to the play in the first scene of act 2 and is described as a "shrewd and knavish sprite." From this initial description, we might infer that Puck is sharp-witted, mischievous, and somewhat unscrupulous. Indeed, we learn shortly afterward that Puck "frights the maidens of the villagery" and "Mislead[s] night-wanderers, laughing at their harm."
Puck is also a servant of Oberon's. Oberon, the king of the fairies, instructs Puck to enchant Demetrius with a love potion so that he, Demetrius, will fall in love with Hermia. Puck, however, in accordance with his role as the play's clown, accidentally administers the potion to Lysander instead. Puck also administers the love potion to Titania to make her forget about the Indian boy, making her fall in love with Nick Bottom, the pompous weaver, whose head he transforms into that of a donkey.
Much of the play's comedy derives from these tricks and mistakes of Puck's, and the moment when he transforms Nick Bottom's head into a donkey's head is one of the most strange and memorable moments in any of Shakespeare's comedies. Puck is also, by the end of the play, a rather likeable character. Directed by Oberon, Puck fixes his mistakes, and the lovers of the play are all matched up with their proper partners. In the monologue which concludes the play, Puck swears to the audience that he is "an honest Puck," and as such, he promises to make amends for any offense that he might have caused during the play.