Yup its Jecht

Floating in the world up high- When Sin destroys Zanarkad and Tidus floats down to the center

Overlooking the city's sky.- Because Tidus is in the sky while he is floating

I come closer to reveal what is real,- He hears Jecht's voice

As, I see you standing in the place where
I must defeat you still- Jecht is standing in the same spot in the dream Zanarkand like he does in the end of the game, as if he was waiting for Tidus to get there soon

But while I'm near, you'll disappear- When Tidus comes closer Jecht fades away and it starts to show a young version of Tidus