Belle could feel there was something more to what and who Lucas was as he appeared to be different than most of the people she had met until now. Be it humans, vampires, or even the pureblooded vampires. It was like he was aloof and yet he wasn't, making others believe he didn't see anything which might have been a butler's nature and the way he noticed every little thing from the way people moved, to the way they looked at things.
To her, Lucas was the most important person after her parents and it might have been because she was an extrovert mostly inside the mansion where she was comfortable while not trying to mingle with others except for a few handful ones.
The man was deadly quiet and polite to people that were somewhere unnerving even though his words were always gentle and sweet to one's ears.