I haven't seen a survivors like this before, and there were a lot of ideas here that were exciting. I like the drag and drop system (I had a really hard time with mine, and yours is really clean), and I like that you can choose what runes you have equipped at the moment. Having a companion was a neat twist as well. As for suggestions:
- It would be nice to have the game pause in the inventory screen like it does in the level up screen, so that the player has time to think over what they're equipping.
- The character speed is so close to the enemy speed that the dash rune felt essential, a higher character speed might help there.
- The bullet spell used health instead of mana, but that might have been deliberate.
The core idea is really solid and I'd be curious to see how you expand the idea going forward!