Puck is first of all a sprite, which is an elf or a pixie, and as such is mischievous and playful, with a naughty sort of playfulness. It is this personality trait that makes Puck--also know as Robin Goodfellow (an ironic sort of name for someone who is always up to no good)--play pranks on the townspeople and frightened the maidens in town: "frights the maidens of the villagery" (II.ii.35). Another personality trait that Puck has is the tendency to mix things up as when he mistakenly puts the magical flower nectar on Lysander's eyes instead of Demetrius's eyes.
This trait leads to results very similar to the results of his propensity for playing pranks. As a result, Oberon accuses Puck of causing the mix up with Demetrius and Lysander on purpose. Yet Puck denies this, thereby showing that while he is mischievous, his personality is not wantonly mean--regardless of appearances to the contrary! One other personality trait that is important in relation to Oberon is that Puck is loyal to and trusting of his master Oberon. Puck willingly does what Oberon says and tries his best to execute his tasks rightly--despite the fact that he is prone to mixing things up.