Lucas greatly dislikes having to do any tests that include Alex because he knows it will be much harder than it needs to be purely due to Alex's behavior. He is not afraid or intimated by Alex at all but is easily annoyed by him and is much more likely to break his composure.
Alex hates Lucas more than anyone else in the facility and will do anything and everything to make every single step as hard as possible for Lucas. Any chance he can get to harm Lucas he will take it without a second thought of punishment. His hatred for Lucas only got deeper when he started using Duncan as a way to try to get Alex to behave.