- Confident and self-possessed, Melissa is smart if sometimes a little scattered. She was the first person brave enough to befriend Milo and is fiercely loyal and supportive of him. She's calm and controlled and loves when Murphy's Law shakes things up.
Melissa is a fair skinned girl with ginger shoulder length hair and blue eyes. Her wardrobe includes a white jacket with rolled up sleeves, a black top and a pink miniskirt. She wears pink shoes with white lining and white socks.
In "Secrets and Pies", it is revealed that Melissa's front teeth are fake.
Confident and self-assured, Melissa is smart, if sometimes a little scattered. She was the first person brave enough to befriend Milo and is fiercely loyal and supportive of him. She's calm and controlled and loves when Murphy's Law shakes things up. She says that a tremendous portion of her self-esteem depends on her school grades ("Sunny Side Up"). She has a tendency to lose her personal belongings, such as her student discount card ("The Undergrounders") or her math book ("The Math Book"). She seems to enjoy betting on the results of Murphy's Law, which she often wins. In "Going the Extra Milo", she bet Bradley and everyone who was interested Milo and Zack would reach school on time, getting their lunches for them and in "Worked Day", she bet Zack that Milo and his father being in the same place would increase the danger, winning a dollar from him.
- Melissa is named after creator Dan Povenmire's youngest daughter, similar to how Isabella on Phineas and Ferb was named after Dan's oldest daughter.
- In "Murphy's Lard", Melissa reveals that she has a fear of rollercoasters after one suspended by a helicopter with a magnet nearly fell on her on the first day of last summer. It appears to be the roller coaster built by Phineas and Ferb in their pilot episode, "Rollercoaster".
- In "Missing Milo", it is revealed that Melissa has a room filled with pictures of Milo, his incidents, and some of his past possessions. She claims that she is using them to better understand Murphy's Law.
- Melissa is one of only seven characters whose eyes have pupils.