Should I fear telling the truth especially against a person I respect very much?
If you respect someone it is only but natural that you speak the truth to them.
Just adulation or sweet talking is not respect.
Truth means expressing yourself with simplicity and naturalness, whatever is in your heart. Not with arrogance, anger or aggression. If you have any doubts you can simply ask. No one will say no to you.
If you speak what you understand is the truth, first find out whether what you assume is actually true or not.