@Xiro ZeanBasically, we just help the GM run things. Review CS's, answer questions - basically pretty much what you've already been doing.
Also some other responsibilities and such as well. In this case, you'll probably take the 'lead' on a mission. As in, you'd be controlling the Environment, NPCs, Gods, and Fiends and stuff for a group of God Slayers on a mission. To prevent a conflict of interest with this, you wouldn't be taking the lead on a mission where Ai or Yuu would be present, of course, just like I'm not going to be GMing anything with Kat present unless necessary for some reason.
Probably also help plan things behind the scenes, as far as plot and stuff goes. At least, that's what it usually is. A Co-GM basically just helps the GM run things, pitches ideas on how to run things, and if the GM isn't able to do it, they go around doing it.
Something like that anyways.
