a girl/gue who is completely oblivious to a guy/girl who loves her and constantly flirts with her but she has no clue and thinks they're just friends and always will be.
Mark hung out with Susan all the time. They went to the out just them and everyone (including their parents) except them knew they were meant to be and were going out. Neither of them could face it because they were being eifbesuns.
Another way is if Mark liked Susan and Susan would talk to him, spill her heart and them become best friends when Mark was secretly in love with her and she was blind to it, or chose to be.