This story is infidelity. The main leads are practical stuck in a relationship that are already broken but they have no guts to go out. Both are main leads are in love with each other it was obvious. They are both happy when they are together or when they see each other but when they with their partners that are trying to fake it.
Obviously, the guy chased the girl because he knows that she is his happiness. She was also the reason why the had a guts to get out of the broken relationship.
As for the girl, she is inlove with the guy but can’t admit that to herself because she is in a relationship and getting married.
Along the way, they probably has faults too but you cannot blame your infidelity because your partner is lacking something. You should be upfront to your partner first and settle things before trying on entering a new relationship with someone else.