In Grand Battle, complete Z Bosses Room with a gold trophy, then talk to Oboro.

In Grand Battle (Episode 2), after winning every tournament in the city you will have access to the Outer World. When there, go to the Z Museum and defeat everyone to be able to go to the Z Boss Room. Once there, get a gold trophy to get the Ray Legend.

When you get to the Z boss room, you will fight (in order), Shiner, Eliza, Izabella, Oboro, then your host, Sensei. Sensei will then introduce your sixth opponent (the special guest), who is a simulation of Hero's father, the first leader and founder of the Z syndicate. He will say that his Robo is obsolete, and the Robo he is using is actually the Ray Legend Robo. It has a much stronger attack and much weaker defense than Ray 01, and will be easy to defeat if you can avoid him and knock him down. Try using all of Rahu 1 but replace his gun with the vertical gun.

To obtain Ray Legand (the Ray from the original Japanese version of Custom Robo for the Nintendo 64), win all five Z tournaments in Grand Battle mode.