Who is the first person to step in Jupiter?

We don't know that person's name, because it has not happened yet. Nobody will likely ever step in Jupiter, because this gas giant as no surface.

Much bellow the clouds of its deep atmosphere, at astounding high pressures and high temperature, may lay an incredible deep hot ocean - not made of water, but made of liquid metallic hydrogen, with some helium...

There is no solid surface to step and walk around, like in telluric planets such as Mars, Venus or Mercury, and other moons.