In this pic Cai is a green unit while Leda is one of the player's unit i.e. blue unit and all 4 main characters can access storage, in another pic Cai is a blue unit. So that means you get to choose one of the 4 main characters in the trailer as the MC or maybe the other blue unit in this pic is a customizable player MC avatar character like Bayleth. Or it's like Fire Emblem Radiant Dawn where you switch between different MCs depending on the story progression.
Nintendo just posted this:
The story begins in a gigantic arena filled with an excited crowd. A man proclaiming himself the Divine Sovereign announces the start of the Heroic Games–and the strength and beliefs of various characters will clash.
Here’s some information on four of the warriors participating in the Heroic Games.
Cai


A young boy from the countryside who joined the Heroic Games, where skilled warriors gather, to help his father. He’s skilled with a lance and can also fight while riding tamed animals.
Theodora


A noblewoman who leads a powerful army. She’s also strong herself, able to dispatch several armoured foes in a single blow. During actual combat, she can launch attacks with the support of her army.
Dietrich




A swordsman characterised by his good looks and long hair. He speaks in a detached way, but his words are often aggressive. During battle, he can use an ability that lets him teleport, ignoring obstacles.
Leda




A woman who captivates all with her alluring dances. Her words reveal a strong sense of conviction. During battle, she can use instruments and songs to call forth Demonic Beasts.




Besides the four shown, other characters also appear in the trailer, engaging in fierce combat. How will the Heroic Games turn out?

Also, there’s someone who looks a bit familiar…
Please stay tuned for more info!