1. Olivia: Olivia is deeply disloyal to her dead brother's memory by falling in love with Cesario (Viola in disguise), a man she believes to be a woman. This disregards her brother's wishes for her to mourn for a year, and could be seen as disrespectful. Additionally, she abandons her duties and social obligations to pursue this romance, which could be considered disloyal to her position and her family.
2. Sir Andrew Aguecheek: Andrew is loyal to the idea of being in love with Olivia, but he is not loyal to her. He is, in fact, easily manipulated by Sebastian, who pretends to be Cesario. He believes he's in love with Cesario (Viola), but easily shifts his allegiance to someone he believes is the same person. This indicates his romantic feelings are shallow and driven by vanity.
3. Maria: While Maria is loyal to Olivia, she is not above using deception and manipulation to gain power. She orchestrates the cruel prank on Malvolio that results in his humiliation. This could be considered disloyal to Malvolio, even if it's all in the name of entertainment for Olivia.
It's important to note that the question of loyalty is complex in *Twelfth Night*, as many characters act out of self-interest, confusion, or even just a desire for entertainment. These characters don't necessarily break oaths or betray in the way that traditional villains do, but they do exhibit behaviors that could be seen as disloyal in some way.