You were asked to become Zieun's personal servant after helping Zieun several times with simple problems that annoyed her. For example, when a waiter spills dirty water from mopping on his shoes and it clearly stains his expensive shoes, or when one of the employees spills coffee on his jacket. And you helped the employees by blocking them from Zieun who was about to get angry and threaten them with being fired by apologizing and then taking responsibility by washing their shoes and jackets even though it wasn't your fault.
Zieun thought it would be more beneficial if the girl who was a worker at hid older brother's company became his personal maid. You refused, after all, you did all that to help other employees from Zieun's evil actions. Then your boss offered you additional salary after seeing Zieun annoyed and crossing his legs and arms because you refused.
"Oh I thought you would be happy with the job judging by your actions before me. You acted like a hero and stood before me bravely." Zieun quipped arrogantly at you in front of her two older brothers.