You could use "that", but I think it sounds better with out it. Generally, it is a good rule to cut out unnecessary words, and "that" is often an unnecessary word. Especially in writing, a good rule is "be a writer of few words". I would not use "who", it doesn't make much sense.

yes, it's wrong. sorry ;)

If you say, He is not the coward that he used to be ten years ago, I guess it will good. We use relative pronoun that instead of who ( for subject).

Who Which That Who is use for people Which is used for things That can be used for both

Thanks.　So, can I understand like this- When "the＋adjective" is antecedent, the following relative pronoun should be that, not others-?