In the new Voting Gauntlet, the in-game help mentions how to battle:
The battle are 3-on-3. The following will be on your team.
A friend, if you have friends registered
A player you have not registered as a friend
So the only guaranteed unit you can choose is your leader.
I thought mine would be Lyn, the first unit in my first party.
However after battling twice it was Sharena both times,
which is confusing because I'm on Eirika's team for the event.
How do I determine who my leader is?