The Comforter
The Comforter which Christ speaks of in John 14: 15-31, and 16:1-33 is indeed the Holy Spirit; it is not a reference to a future prophet, for Jesus said He was the First and the Last.{Revelation 1:8}
"I am the Alpha and the Omega", says the Lord, "the one who is and who was and who is to come, the Almighty." The future coming, which is referred to so many times in the New Testament, is not to another prophet; rather, it is to Christ's Second Coming.
When John was on Patmos Christ spoke to him, repeating that He and He alone was "the first and the last, the one who lives" (Rev.1:17-20), the One who was Resurrected.
Further, Christ mentions to His apostles that the Comforter would always be present with them on earth. (John 14:16-17)
If the Comforter, a paraclete, is to always exist within the faithful, then how can it possibly be an ordinary man?
Outside of this, there is nothing in the OT or NT which suggests that a new fulfillment of God's Law had to be made; Jesus was the Fulfillment of the Law, as He was God Himself.
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Galatians 2:20 Yet I live, no longer I, but Christ lives in me; insofar as I now live in the flesh, I live by faith in the Son of God who has loved me and given himself up for me.