In contrast to Sunny, Anti-Sunny has a gloomy, dark attitude and terrible sense of humor. She is extremely jealous of her counterpart. Anti-Sunny acts like a spoiled brat towards her parents, and has a huge crush on Foop, and teases him. She hates regular fairies and wants to destroy them all, ruin the lives of miserable children and take over the universe. Sometimes, Anti-Sunny gets upset over certain things and throws a crying fit. Anti-Sunny's voice is deeper than her regular fairy counterpart, and sounds more monotone.
However, as of the episode "An Unlikely Team-Up", Anti-Sunny put her differences from Sunny aside and becomes her friendly rival.
Anti-Sunny resembles Sunny, but she is shaped like a cube rather than a sphere. She has bat wings and a black crown like all Anti-Fairies. Her pajamas are slightly darker blue than her skin, and they have a skull symbol on them.
While Sunny was born from Neptunia, Anti-Sunny was born from Anti-Astronov instead.