How do you choose units to use in the Voting Gauntlet?

Question

In the new Voting Gauntlet, the in-game help mentions how to battle:

Battle Format:
The battle are 3-on-3. The following will be on your team.

Your Leader
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?

Answer 1

Although the news notification from Nintendo says:

The leader is the ally placed in the leftmost slot of the first team

It appears that the ally you bring into battle is actually the leftmost slot of your currently selected team.

I have done 3 battles in the gauntlet now and can confirm that the leftmost hero was Sharena twice, then i moved my selected team and got the leftmost unit in that team instead (in this case a Lyn)