This two is too similar i can't explain. Maybe it's the acting storyline and how deep both characters are.

Both are a kmovie about a female killer, the two worked in a violent job and ended up killing everybody. Kill BokSoon has a more complex storyline with other characters, while ballerina has an emotional thing on it.

The relationship with a "dead" person kinda reminds me of Something in my room. The characters personality and dynamic are also pretty similar, the quiet and calm guy with the guy who is all the time after him. The sad and angst feelings, both with tragedy plot and the passionate romance between the leads is a good complement of both series.