Sub-Personas (サブペルソナ, Sabu Perusona)? are a unique mechanic in the Q series.
Persona Q / Q2
Due to the location in which two Wild Card users are brought together, the normal power of the Wild Card is altered. No longer can the Heroes change their main Persona, however they and everyone else are able to equip a Sub-Persona. As Sub-Personas do not have stats or resistances, everyone's stats and resistances are determined by their main Persona. Sub-Personas, however, do give a bonus to HP and SP which gets renewed at the start of every battle; careful management of HP/SP spending can allow the player to progress without running their party dry.
All skills from a Sub-Persona are also added to the equipping characters' skill repertoire, granting skills their main Persona normally cannot learn, and adding a degree of customizability to each party member's roles. Notably, Sub-Personas can also learn or inherit passive skills like Resist/Absorb [Element], which can be used to cover the main Persona's inherent weaknesses.
Main Personas can no longer be fused; instead, the fusion mechanic is performed exclusively on Sub-Personas. Each Sub-Persona can hold a maximum of six skills. They also hold a hidden inheritance type which determines which skills they can or cannot inherit; frequently a Sub-Persona will not inherit skills of the element opposite to what they specialize in.
Just like Personas used by most of the cast, Sub-Personas are usually classic mythological figures commonly seen in the Shin Megami Tensei series, although DLC Personas such as Kaguya Hime are exceptions.
If an equipped Sub-Persona is 6 levels or more below the character's main Persona, all skills suffer a penalty, reducing their effectiveness by 10%, with the penalty increasing with each level to a maximum of 50% at 10 levels below.