This isn't an MMO, there isn't any individual character to develop.
A good analogy is you have the monopoly board game, you invite your friend over for a game. You pick a car, your friend picks a thimble from your box of selectable game pieces. All the pieces that you own.
You play for a gazillion hours and finally end a game. You had fun, and your friend goes home. The board game is still yours after the game ends, your friend don't get to take your board game home.
At most, if you started a new game with your friend, s/he would have been able to customize one of the main heroes, but the game is being hosted by you.
