Setting Teams and Leaders

Tyranys

How exactly is your arena defense team determined? Also is there a way to set your leaders different for when your allies use them?

Thirdly how is your leader determined for use by allies in neutral instances?

DanTom

Your defending team is set as the last one you used in arena. Any upgrades you add to those characters will be present in the defending battles though.

I believe allied heroes are the ones that that ally last used on that mission, but I'm not 100% sure on that. Some just seem to be random heroes.