Well, carved foam immediately comes to mind. Having never worked with it though, I can't really offer any input on how well it would work, though from what I've read I imagine it'd be reasonably light and sturdy. :/ Or, if you figure out something better but heavier, you probably wouldn't have to make the body out of the same thing, due to it being covered all the time anyway, just the arms, legs, and head. Hopefully someone else can be more helpful ^^;
F! Ardyn Izunia - Final Fantasy XV - Done!
Logix Fiscario - Atelier Escha & Logy: Alchemists of the Dusk Sky
Meruru Rede Arls - Atelier Meruru: The Apprentice of Arland