Let the baseless speculation begin!

I feel the grey haired girl in the trailer will be important later on and a potential party member.

Most of what we saw in the trailer is the game's prologue. Starting with the group hunting Shiva and flash-backing to the lead's childhood with Joshua.

Joshua is the main antagonist of the plot, but not the main villain.

The game revolves around kingdoms in conflict, with their powerbase being people who are the summons. They are exceedingly rare by the main time frame of the story.

The lead is granted a boon by the summoners which extend his battle style. This is why the summons/summoners are so important because they can potentially increase the power of the nations army.

The game will have a party system but the other members will likely serve only as a support unit for the lead.