Mason defects when Emperor Evulz refuses to give him a free limousine. However, La Résistance doesn't want to give him a free limousine either. Mason ends up creating an alliance called "Free Limousine for Mason", but can't get anyone to join.
Enforced: Because of his reluctance, the Empire tries to kill him, forcing him to switch sides.
Lampshaded: "Could you do something about the outfit?" "It's the only set of armor I have". "Come on! File down the spikes, get a paint job, something!" "Sorry".
Mason's commander, suspecting that his loyalty isn't absolute, deliberately engineers a Secret Test of Character in the guise of a Moral Event Horizon to see whether or not he'll defect (fully expecting that, if he does defect, he can kill Mason before he escapes).
King Sable and Queen Satine decide to align themselves with the empire in order to protect their kingdom, despite suspecting that their daughter will leave and start supporting the rebellion openly.
Conversed: "That one's too nice, he will never last running with the forces of Emperor Evulz".
Despite switching sides because he could no longer stand by what his peers were doing, Mason finds himself still judged by their behavior, as well as his past misdeeds. The camadarie he once shared with his comrades is gone, the bonds broken by his own decisions, and he can't forge new ones to replace them because nobody trusts a traitor. Ultimately, Mason becomes a Martyr Without a Cause, seeking to atone for his mistakes by sacrificing himself for his ideals.
Princess Velvet quickly finds that her rank hardly matters in the rebellion, save among those hoping to make her a puppet ruler or otherwise take advantage of her — something which becomes somewhat of a moot point after her parents disinherit her. She struggles to adjust to the harshness of life as a renegade underground, and questions whether her decision to defect was really all that smart, ideals aside.
Though distrusted by the bulk of the resistance's forces, Mason manages to prove himself to a few members and forges a new group of True Companions, their bonds strengthened by shared ideals. This reinforces his newfound alliegence and assures him that he made the right decision turning on his former allies after they crossed the line.
Despite becoming a Fallen Princess, Velvet buckles down and becomes a crucial figure in the rebellion, inspiring others through her sacrifices and her absolute refusal to become anyone's puppet.
Played For Laughs: The Rebels are just as bloodthirsty and horrible as the Empire, but Mason doesn't mind, the real reason he defected were those awful Imperial Army combat rations.