Reno is actually Mamoru from Ambassador Magma

[Toshio Tenma]

Considering Tezuka Star-System which each character is a sort of "actor" which appears in several series in different roles, I've come with the conclusion that Reno from the 2003 series is actually Mamoru from the Mangá Ambassador Magma (Magma Taishi).
Look:

Do you notice the resemblance?


Reno's robot desguice also give it away, it's actually Gam from Ambassador Magma:


Also, it was not like Gam changed gender in the 2003 series.
When the robot family composed by Magma and Mol was added in the 2003 series, Gam was not necessary since Mamoru played the both role of Reno and Gam (in Reno's robot desguice, despite not being called like that).
So, they made a robot girl to replace Gam, rather than changing his gender.
Gam was choosen as Reno's desguice, because in Magma Taishi the character Gam was himself based in Mamoru as a request of Magma who was touched by the boy's bravery and wished a son with his looks.

Funny is the fact that Reno/Mamoru never interact with any character from Ambassador Magma in their versions of Tetsuwan Atom 2003.

[F-Man]

Very good observation! You're right that his robot disguise gives it away.

[Toshio Tenma]