Where is Mr. Fuji in Pokemon Yellow?

He is on the top of the Pokemon Tower. you can get to him after defeating Marowak that is the ghost of the tower. after talking to him he will say follow me at my house, he lives south of the tower before the Poke mart.
He is at the top of Lavender Tower. You're going to need a Silph Scope from the Team Rocket Hideout in Celadon if you want to actually rescue him.
You need to beat Team Rocket in Celadon City's Game Corner. Touch the poster in Celadon's Game Corner and enter the the basements. Once you have beaten Giovanni, the leader of Team Rocket, you'll get a Silph Scope as a reward.

With this Silph Scope, you can unveal and defeat the Ghost who is blocking your way to the top floor of the Pokémon Tower (Lavender Town). On the top floor, there will be a man who will give you a very useful item.