this Mr Johnson?

"this" is often used to replace the definite article in English - eg :
So I walked in and there was this man just sitting there...

I can't find any examples of it used like this with names, though it's not impossible to imagine it :
So I walked in and there was this Mr. Evans sitting there.

It's more often used with its normal demonstrative meaning - ie referring to something already mentioned. So it has the meaning "The same Mr Johnson that we have been talking about." Here's Jane Austen in Pride and Prejudice :

I wonder that the very pride of this Mr. Darcy has not made him just to you!

Other examples :

So who is this Mr. Johnson?
This Mr. Johnson is OK!

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thanks~