Ganta cares a great deal for his friends and allies. It is later revealed that he actually has strong feelings of love towards Shiro and that the feelings are mutual.

How did Shiro know Ganta?

In the past, Ganta knew Shiro, because his mother was on of the chief researchers at the laboratory, extremely involved in the inhuman experiments the girl was subjected to. Gantas cared a lot for Shiro, and she liked him a lot too. They pretended to be each other's hero, "Aceman".

Was Shiro the red man?

Shiro is the Red Man, AKA Wretched Egg, or more precisely, the Wretched Egg is a split personality of Shiro's. The outfit of the Wretched Egg is derived from Shiro's personality where Shiro impersonates Ace Man to act as Ganta's hero. So yes, Ace Man did have direct correlation to Shiro and the Wretched Egg.