Such a brother must be stopped. Going about introducing a sister, as your future wife when she has not given her consent is not only bad but also manipulative.
As a believer, you have a right to be led by the Lord as to who you get married to, and this must be respected by whoever desires your hand in marriage, no matter the strength of his conviction about you.
Whoever goes to that extent, promoting himself as your would-be-husband is not only an impersonator. He must be confronted.
If confrontation does not work, however, he has to be reported to his pastor or someone he submits to. If he does not have anyone he submits to, he must be carefully avoided.