This entire section of Scripture (John 13-17) is Jesus in the upper room with the twelve apostles. He is speaking to them directly and giving them instruction about his coming death and what will follow.
Regarding the Helper (e.g. “Spirit of truth” or the Holy Spirit) specifically, Jesus later in this context defines more of how He will help them: (14:26) But the Advocate, the Holy Spirit, whom the Father will send in my name, will teach you everything and will cause you to remember everything I said to you. (16:12-13) I have many more things to say to you, but you cannot bear them now. But when he, the Spirit of truth, comes, he will guide you into all truth.