One of Puck's main character traits is mischievousness. We learn about his mischievous character when we first meet him in the second act. Puck's fellow fairy asks Puck to verify if he is "that shrewd and knavish sprite / Call'd Robin Goodfellow" (II.i.33-34). The term "shrewd" can be translated as either intellectually sharp or malicious, meaning having a tendency to want to cause harm (Collins English Dictionary). The term "knavish" can be translated as "mischievous." Therefore, Puck's fellow sprite is accusing Puck of being an intellectually devilish, or mischievous little fairy. Both Puck's fellow fairy and Puck himself go on to describe different antics Puck has been known for, such as scaring the maidens in the village (35).
Beyond being mischievous, we also know that Puck is very loyal. He is a very loyal servant to Oberon and is Oberon's court jester. We especially see Puck's loyalty when he very willingly and very speedily carries out Oberon's requests. For example, when Oberon asks Puck to find for him the magic flower as quickly as he can, Puck very obediently replies, "I'll put a girdle round the earth / In forty minutes," meaning that he'll circle around the earth to find the flower in forty minutes time, which is quite speedy (178-179).
We also know that Puck is a very compassionate fairy. We see evidence of his compassion when, after Oberon tells him to enchant the Athenian man who is disdaining a fair maiden, meaning Demetrius and Helena, Puck finds whom he thinks is the couple and feels pity for the way the man is treating the lady. When Puck sees the couple he believes Oberon is referring to, he sees that the maiden is lying far off from the man and believes that it is because the man hates her and is being unkind to her. We see Puck express sympathy for what he believes are the maiden's feelings in his lines, "Pretty soul! she durst not lie / Near this lack-love, this kill-courtesy" (II.ii.76-77). We further see him express compassion for the maiden when he calls the man a villain, as we see in his line, "Churl [villain], upon thy eyes I throw / All the power this charm doth owe" (78-79). Hence, not only is Puck devilish, he is actually a "devil" with a compassionate heart.