Faithful (adjective)

1. Adhering firmly and devotedly, as to a person, cause, or duty.
2. True to one's word, promises, vows, etc.

Origin:

From old english gefæġfull ("true, faithful"), from gefæġ ("peace, agreement") + -full ("-fu.l")

Examples:

1. He is a faithful husband to his wife.
2. The dog was a faithful companion to the man.
3. She was a faithful follower of the leader.
4. The faithful customer always comes to the same store.
5. He was faithful to his promise and kept his word.