faithful : 多指对人对事或对诺言、誓言的始终不渝，侧重在任何情况下都绝不改变。
- a faithful friend
- They are faithful supporters of the Labour Party.
- His faithful old dog accompanied him everywhere he went.
loyal : 一般指对祖国、领袖的忠诚或对事业、原则、誓言的忠实，不动摇。
- Jack has been a loyal worker in this company for almost 50 years.
- When all her other friends deserted her, Steve remained loyal.
- She's very loyal to her friends.
constant : 侧重指思想状态的稳定或信念的坚定，也指爱情或感情等的忠贞不渝。
- Fred had been her constant companion for the last six years of her life.
true : 侧重个人感情上的忠实和不动摇。
- She has vowed to remain true to the president whatever happens.
- He said he'd repay the money the next day, and true to his word (= as he had promised), he gave it all back to me the following morning.