Fake / Contract Relationships - OSF

1 - Episode 3 - 5

1* - The ML has a secret crush on the FL. Then he finds out the FL is secretly dating a co-worker who is engaged to be married to another woman. He tells the FL about it (and also confesses his own feelings) and when she eventually asks what she should do, he offers to be her boyfriend to get back at the two-timer. She initially rejects the offer, but they end up going with it after she tries to break up with the two-timer and it doesn't go well. 

1 - Fake dating to appease the ML's grandfather / to trick the FL's rude "friends".

1 - Married in an online game (the ML is aware of the FL's gamer identity, but the FL doesn't know that the player she marries in the game is the ML at first).

1* - Marriage on paper so the FL can use the ML's nationality to aid a mission. They also use it to their advantage later on to escape danger. When the marriage happens, the FL cares about the ML. However, even though they never discuss it, it's understood that they are not in a relationship. The FL's heart belongs, first and foremost, to her cause - fighting for Korea's independence. 

2* - The ML already knows the FL likes him and rejected her, but also has a bit of a soft spot for her and doesn't publicly deny it when others mistake her for his girlfriend because he doesn't want to embarrass / publicly humiliate her. When they both end up at a meal with their two families, the elders try to set the ML up with the FL's cousin and the FL up with the ML's cousin. The ML's cousin avoids it by saying that he heard that the ML and FL are already dating. Due to the situation, the ML isn't able to deny it. He's forced to say that they are dating, and he and the FL agree to fake date for a while with the intent that they break up after a sufficient amount of time has passed.

2 - The marriage between the leads is bestowed by the Emperor at he Dowager Empress' suggestion. The FL does not object because she already has some feelings for the ML based on a childhood interaction, but her objective at that point is to "subdue" the ML. The ML is afraid of and basically disgusted by the FL. Eventually the FL gives the ML a divorce letter dated three years in the future to change his mindset. The ML believes that their marriage is just for show until the divorce date.