What are “a” and “certain” adding in meaning to the phrase “a certain Mr. Ripley”?

Question

Consider the following sentences:

I had my identity stolen by Mr. Ripley.

and

I had my identity stolen by a Mr. Ripley.

and

I had my identity stolen by a certain Mr. Ripley.

In what sense do these 3 sentences differ from one another? How do a and certain modify the middle and last sentences?

Answer 1

I had my identity stolen by Mr. Ripley.

1) This suggests that you and the reader/listener are familiar with Mr Ripley, or that his identity isn't of particular importance.

I had my identity stolen by a Mr. Ripley.

2) Now we're starting to place emphasis on the identity of the thief in question. It suggests you've done some research on the problem and that you've discovered the person.

I had my identity stolen by a certain Mr. Ripley.

3) This is an extension of 2. It is a preface to additional details about the thief. It would naturally be followed by, "he lives at such-and-such a place," etc.

Answer 2

"A certain Mr Ripley" acknowledges that the audience is not (necessarily) familiar with the Mr Ripley in question. It also implies that the Mr Ripley in question has been ascertained, rather than "a Mr. Ripley" which rather implies that the name is all that has been discovered.

It may also be used ironically, of course, when the identity of the Mr Ripley in question is not in doubt.