Below contains some spoilers to explain why this manga isn't depicting a case of the Stockholm Syndrome, which would understandably put many people off. If you don't want to see any spoilers go to TLDR.
The girls life has been bad; So bad that she get's abused by her parents at home, her teacher and her fellow students at school. She doesn't even have a lot common sense, such as the knowledge on the existence of a pause function on videos online due to her circumstances. Being "Kidnapped" meant she is more happy than when she was not kidnapped. And this isn't even proper kidnapping. The girl can probably run away at anytime; she isn't confined to a room, nor is she chained. The "kidnapper" is kind to her, cooks her meals, let's her watch TV and even takes her out sometimes. Moreover when you hear the word "kidnapping", you'll probably think of a person forcefully shoving another person in the back of a truck. This was not how she was kidnapped. In other words, this isn't proper kidnapping so don't get a preconceived idea that this is a manga depicting a case of Stockholm Syndrome since this manga doesn't actually show it. A real case of the Stockholm Syndrome is a lot more messed up.
What this manga actually shows is someone showing kindness to a girl that has had a really unfortunate life and this girl finally experience what happiness is. Throughout the manga you learn more about the girls circumstances and a tiny glimpse into the life of the kidnapper, though it's still mostly shrouded in mystery. We see that their relationship is a complicated one, with both of them having their fair share of troubles.
TLDR;
In summary, just see this manga as a slice of life manga about a girl experiencing happiness for the first time in her life from someone, meanwhile the manga delves deeper into their circumstances.
