The Spirit of Truth

"And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever; Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you." John 14, 16.17

Jesus here says that this "another Comforter" is "the Spirit of truth". Now, read who is this "spirit of truth" according to the Spirit of Prophecy:
" Christ was the spirit of truth. The world will not listen to His pleadings. They would not accept Him as their guide. They could not discern unseen things; spiritual things were unknown to them. But His disciples see in Him the Way, the Truth, and the Life. And they shall have His abiding presence." SW, October 25, 1898

So, Christ is "the spirit of truth" not someone else. He is our Comforter who, after His ascension in heaven comes to us in another spiritual form while in His human body is on the right side of the Father. Therefore He is called "another Comforter".
"The Saviour is our Comforter. This I have proved Him to be." 8MR 49

"Let them study the seventeenth of John, and learn how to pray and how to live the prayer of Christ. He is the Comforter. He will abide in their hearts, making their joy full. His words will be to them as the bread of life, and in the strength thus gained they will be enabled to develop characters that will be an honor to God."  RC 200