In regards to A flirting with others to test the MC, what if MC was prone to insecurity and low self-esteem? I can't imagine A would be particularly sensitive to MC's feelings
A finds it enjoyable to test people and see how they react to buttons being pressed. That being said, once they had seen how the MC reacted to them flirting with someone else and they'd realize that it would only make the MC feel bad rather than spur them on (is that the right way to put it?), they wouldn't find it a fun endeavor anymore either. They wouldn't knowingly continue make the MC feel insecure in regards to their relationship.
Thank you for the ask!

















