I don't really know kanji, but since nobody else replied I decided to do some looking around to see if I can help. As far as I can tell, 素 can mean something like "plain", and 体 can mean something like "body" or "corpse". Also, looking up the word on Google, I'm seeing things like dolls and action figures, and when I pair it with other names, like "素体 マリオ" (Mario), I get figures of the character.
So my guess would be something like "empty doll", which the player fills during the character creation. I think "vessel" sounds more natural than "empty doll", so that's what I'd use. I also looked up the Langrisser characters, and I think the mobile game calls both Sigma and Lambda "man-made vessels" as well, so I think that should be it!