Being mischievous refers to someone who is playing pranks or causing trouble in a playful or slightly malicious way. The opposite of being mischievous would be a person who is serious and well-behaved, perhaps someone who follows all the rules and rarely steps out of line. There are a number of antonyms that can be used to describe someone who is the opposite of mischievous, including responsible, sensible, serious, mature, obedient, and respectful. Other words that can be used to describe those who are not mischievous include well-behaved, disciplined, reliable, and trustworthy. These words highlight the characteristics of someone who is more likely to follow the rules and do what is expected of them.