The scenario is not really a fresh concept in itself : a surnatural being coming to earth and relying on one human to get used to "earth life" and then, they obviously fall in love.
But a few details made the difference here.
-  the FL is not helpless, just unaware of certain rules. And she learns fast, is always very careful of how she acts around others and tries her best not to be a burden. I really like the fact that they wrote a very sensible and intelligent yet bright character for her. She's independent and likes to explore on her own, just like a cat.
What made her character good is also Gia Ge's amazing performance.
I think she did a really detailed job with her body language, micro expressions and sounds to act like a human-cat without making it weird either. For example the way she moves her head when she pays attention to something is more cat-like than human-like but it's too subtle to feel like a forceful detail. 

- I also like the fact that they managed to describe very accurately the world of an introvert (the ML) without making him a lonely and hurt wolf. He has a complicated past but accepts the situation and I think that makes him more realistic than the typical forever-wounded-but-saved-by-love ML.
He appreciates his independence, is an easy going yet complex character and the bromance with his best friend is really natural and fun to follow.
But none of this would have been possible without the performance of Ren You Lun.
Everything in the way he talks or moves totally embodied the character . And that's why this already-seen concept works here. The story is impossible but the characters are so well portrayed by the main actors that they make it believable. I didnt doubt a second that Ren You Lun was in fact Chi Yan, which made him realistic and therefore made the whole story "believable".

There are a couple of downsides, some parts were a little artificial. As a person who has felt abandoned many times and who had doubts about being vulnerable again, it wasn't really clear how the ML suddenly got over his worries  and decided to date the FL .
I also really liked the twist towards the end but the last 4 episodes had a little bit of a weird rhythm because the romance is pretty much resolved at episode 20/21 and then the rest is a rush through the other plot.
Some solutions brought towards the end were also a little bit forced but this is a c-drama so compared to others, it still flows pretty well.

Overall this is a very sweet romance with a very good performance and chemistry from the leads. I almost didn't use the +10sec button, which is RARE with c-dramas.