[Robert B. Mercer]

My pleasure.

Also, in my experience, you would call someone "Robert Mercer" or "Mr. Mercer," not Mr. Robert Mercer.... you use Mr to be formal, so in those cases you would not use the first name. If you use a first name, you are not formal, so do not use Mr.

"Rannma, this is my brother, Tom Smith."

OR

"Rannma, this is my teacher, Mr. Smith."

EXCEPTION

If you introduce one teacher to a peer, to a different teacher, to an audience of peers, you use Mister... because you use the formal name when you talk to your teacher. BUT you know that your teacher might want to be friendly with the other teacher and use the their first name,
THEN I would say,
"Miss Thompson, this is my English teacher, Mr. Tom Smith.... Mr. Smith, this is my math teacher, Miss Susan Thompson."

This is especially important because perhaps I do not want to use my first name... if you introduce me to your father as "Father, this is my teacher, Mr. Mercer"... I can just say:

"Hello Mr. Rannma,"
OR
I can say "Hello Mr. Rannma, please, call me Robert."