Is there a way I can easily set up inventory items to be useable on the main character? Or do I have to set up a code for each one?
You have to set up an if (player.activeinv == X) code for each item in the 'Character-Interaction-Use inventory on character' section fro EGO, as you do for every character (hotspot, object, etc).
You could add a 'Use on self' button to your inventory GUI, and run it from there rather than having to open the inventory, select the item, close the inventory and click on EGO, but you'd still have to write an interaction for each item.