Fusion is performed using either the skill Comback or via Geber's Laboratory. Beast fusion becomes available either when Mattel learns the Comback skill or when the party reaches Dolurae. Fusion accidents and skill inheritance do not occur, and there are no special mechanics involved in fusion aside from the use of the group system in place of a demon's race.
In line with the weather calendar aspect of Persona 4, each month has days when fusing in the Velvet Room will grant the fused Persona a bonus. All of the fusion forecast bonuses for Persona 4 Golden are listed on Calendar in Persona 4 Golden.
The types of bonuses are:
Stat Increase: Increases random stats by +2.
Learn a Skill: On some dates, fused Persona can learn a predetermined skill regardless of their level. On other days, they can learn a skill of a certain attribute (Fire, Ice, Elec, Wind, Recovery, or Support) based on their level range.
Social Link Bonus EXP: Further increases the Social Link bonus EXP based on what rank the Social Link is.
See Fusion EXP Bonus for the base amount without the fusion forecast bonus
Skill Change: Random skills will change into other skills from the same numerical rank group it is part of. The player can reject the skill change.
Typical two-Persona fusion is unlocked in the Velvet Room partway through the first Palace. Fusion results are determined by the Arcana and level of the chosen Personas. The player is able to choose which skills are inherited, though some skills are unique to a particular Persona and some skills are impossible for a given Persona to inherit. For example, a Persona with Ice inheritance type will not be able to inherit Fire magic skills. In Persona 5 Royal, the Trait of a Persona can also be inherited by the fusion result, except for the traits of DLC Persona.
Group fusion of three or more Personas becomes available on reaching the Strength Confidant Rank 5, with recipes unlocking from reaching maximum rank with some Confidants, earning key items in Mementos missions, or progressing through the game. One group fusion result is only available in New Game+. The Velvet Room also eventually unlocks Persona itemization, which can give equipment or skill cards, and Persona strengthening via sacrifice of one Persona to provide experience and randomized inheritance of one of the sacrificed Persona's known skills. Late-game unlocks Network Fusion, which requires connection to the internet and in which once daily a standard fusion may be performed with one of the two Persona being provided by a randomized selection of other players' Personas. This can be a way to obtain skills or Traits not yet available to the player, or which are only obtained through Fusion Accidents or Fusion Alarms.
