Not far into the game, you will learn that your main character, as well as all of the others, can equip another Persona, in addition to their main one. These Sub-Personas, as they are called, function a little different than the multiple Personas used in previous games. Instead of giving a plethora of stats and resistances, Sub-Personas only give three main things in Persona Q: more HP, more SP and additional skills. There are no restrictions to which Sub-Persona can be equipped on any given character, save for Zen/Rei, who cannot equip any at all.
Battling with Sub-Personas
There’s nothing special you have to do to summon your Sub-Persona in battle, as you just need to use one of the skills that you have on that particular Sub-Persona. When you first equip one, you will notice on the character’s portrait, that their HP and SP bars have an additional white component added to them. These can be treated as free HP/SP, as after a battle they will be replenished.
As an example, let’s say you have 150 SP by default and equip a Sub-Persona that grants an additional 50 SP. You use a skill in battle that takes 35 SP, so you’re left with a total of 165 SP. If you complete the battle without using any more SP, then that character’s SP will be refilled to its max after the battle. If you use the same skill for an additional 35 SP, then your character’s SP will dip into their actual pool and not be refilled back to full after the battle is over (the 50 SP will always replenish, though).