You could try having an open and honest conversation with them about how it makes you uncomfortable. Explain that it's a fictional and personal interest that you'd prefer they respect your boundaries regarding.
One simple way is to just ignore it as long as it's not causing any real harm. Sometimes people get excited about things and it might pass on its own.
It might be that they are really attached to you and think highly of you. Maybe they have a lot of free time and focus too much on you. Or it could be that they are trying to figure out how to have a better relationship with you and go a bit overboard.
Maybe they find the storylines and art styles really engaging and different from what they're used to.
It could be that the diverse themes and characters in manhwa appeal to their interests. Or perhaps they enjoy the escape and entertainment it provides from their daily routine.
Maybe they see something special in you. It could be your kind heart, or your unique talents. In the novel, perhaps you represent a new hope or a fresh start for the family.
It could be that your actions or behavior have made a deep impression on them. Or perhaps they see something in you that they really like and can't get enough of.
Maybe they see something special in you that catches their attention.
Perhaps they are drawn to the drama, romance, or adventure elements present in the manga. It's hard to say exactly, but there could be multiple reasons for their obsession.
It could be that your actions or personality have made a deep impression on them. Or perhaps there's some underlying plot reason within the manhwa's story that explains their obsession.
It could be that your actions or behavior have made a deep impression on them. Or perhaps they see something in you that they admire or find fascinating.