be
remain, stay
soldiers who stayed faithful to the king

extremely, very
He has been a very faithful friend to me.

absolutely, completely, utterly

to
He had remained completely faithful to his wife.

be

extremely, fairly, very, etc.
remarkably
completely, entirely

to
The movie is faithful to the original novel.

Faithful is used with these nouns:
adaptation, adherence, Catholic, Christian, companion, copy, disciple, dog, follower, following, friend, husband, reader, representation, reproduction, servant, service, steward, translation, version, wife