Im aware that when i help someone in multiplayer, im playing his character. Do i get any benefits like gold or items?

 

i loved the fact that i can drop-in and drop-out whenever i want, but is there a mode where i can use my own characters to play with others?

Characters stay in their own campaign and aren't imported or exported across separate campaigns. As such, even if you got extra benefits like gold or items, it wouldn't matter - if you went and hosted your own game, you'd still be making new characters.

 

In short, multiplayer is only about co-op, there is no persistent mode.