Tensei Brave battle party setup comes down to one rule: the party that fights with you is the protagonist plus the companions you have already recruited, and they travel with him.
What is actually in your battle party
Your party fills up as you recruit: the protagonist holds the first slot and every heroine who has joined takes a slot of her own, while the slots of companions you have not recruited yet are shown greyed out. To fill the empty slots, use the recruit allies guide.
Reading the party on the field
The bottom-right HUD shows one portrait per party slot, each paired with the key that triggers that member's action: SPACE for the protagonist's action, and J / K / L / H for the companions' abilities. Because each key is bound to a slot, the portrait printed next to the key tells you exactly which companion that action belongs to.
Watching your party's condition in battle
In battle the right-hand column lists each member with their portrait, their current HP and MP, and small icons for their weapon and their current condition. Read those numbers before you commit to an action, because the condition icon tells you when a member is not acting normally. Items can be used from the same command set as attacks and skills, so a consumable can be worth spending a turn on instead of an attack. What those learned skills do, and how a character earns them, is covered in the level up and unlock skills guide.
When a companion goes down
Party members can be knocked out and left behind. Fallen allies are brought back at the church in town, which also cures status ailments, while the inn restores HP and MP and the item shop sells items and weapons. After a path opens, revive and heal the party before moving on, because the fight that opened it leaves them down.