Cassandra is permanently gloomy. Their teacher even referred to her as 'our little ray of sunshine' in the first episode. However, she does find some pleasure in her funny, cunning, sarcastic comments, which are always there when someone makes a mistake. That said, Cassandra is very loyal to her friends, and often acts as a mentor for Hercules in times of need, bringing patience to Icarus's 'Icarus-ness.' Cassandra is not opposed to lying to get out of things she would rather not do (dates with Icarus being the prime example of this) and seems to be opposed to motherhood, though it appears that she might make a good one. Cassandra is also very anti-social.
Although she is gloomy and anti-social, Cassandra can also be very caring, even to Icarus. She is shown to usually be more upbeat when she is around Hercules though no less sarcastic.
Powers and abilities
Cassandra has the ability to see the future (usually disasters), but is cursed so that no one ever believes her. When Cassandra has a vision, her eyes turn green and swirl around. A running gag is that when Cassandra makes sarcastic comments, Hercules and Icarus listen, but when she has visions, they rarely believe her. In Greek Mythology this is called the Cassandra truth.
Cassandra even pointed this out in the episode "Hercules and the All Nighter", where she and Icarus switched personalities as a side effect of sleep deprivation.