Coach

I see, thanks for answering.

such a useless question

It is an interesting question especially as top players have coaches that are not as strong as they are themselves.

Coach can only guide you, watch what are you doing, supervise your work. He can't make the progress instead of you. It is you who must work, analyse position, calculate. Coach is there only to watch everything by side.
Very strong players usualy hire good analysts that analyse and prepare openings for them. Sometimes they also like to hear a second opinion regarding some line or position.

@PetarBosnjak I agree that student's own work is the most important part of one's success in chess, BUT wouldn't you agree that coach can make one's progress way faster. Example: I buy some chess books, I read them every day, I solve random chess puzzles, I analyze my games but it's hard to find my mistakes and I don't know what are my weak/strong sides, so I spend hours of time on analyzing my games and searching for my weak spots. This way I will make progress however if I hire a coach who provides me exact books/chapter that I need to read, he will instantly see my mistakes in the games and tell me what are my weaknesses, he will tell my general opening plans/ideas, etc etc. What I mean is that a coach can make one's improvement FEW TIMES FASTER.