It could be her past that makes her mysterious. She might have had a difficult upbringing or some traumatic experiences that she hasn't shared with the CEO yet. Also, her behavior might be a bit unpredictable, like she suddenly disappears at times or has strange habits that are not easily understood.
The problem with 'You Duke' manga might lie in its lack of originality or poor world-building. Also, the dialogue could be weak, making it hard for readers to engage.
First, you need to have a solid foundation in both the source and target languages. You also have to be familiar with manga culture and terminologies. A degree or certification in translation studies can be helpful.