Allows you to follow the leader's attack command with an attack from the linked materia. (Does not consume ATB charges or MP.)
Description
Synergy materia is a 
Support Materia in Final Fantasy VII Rebirth. It can be linked with attack 
Magic Materia to cause the user to follow up an attack command from the currently controlled player character with the first tier ability from the linked materia, at no MP or ATB cost.
Synergy materia returns from Final Fantasy VII Remake, but in the form it appeared in Final Fantasy VII Remake Intergrade with one change: the follow-up spell is half as strong in comparison to when used via the Magic command, whereas in Remake and Intergrade the follow-up spell was full-powered.
Though Auto-Cast Materia also exists, it has a different role in that it causes the party member to act independently, and does not eliminate MP costs.
Obtained[]
Chadley sells synergy materia in his Materia Development at Cosmo Canyon Region for 80 data points.
Use[]
Synergy materia causes a party member to cast the first level spell from the linked attack magic materia at 50% potency after the player-controlled party member performs an attack that generates a synergy charge, and the synergy-user is not already using another ability. For example, linking synergy with Fire Materia will cause an idle synergy-wearer to cast 
Fire on the enemy attacked by the player, but nothing will happen if the ability used did not generate a synergy charge, such as with Cloud's Disorder. This can give the synergy-using party member a powerful attacking output, allowing them to spam spells and at no cost to themselves.
Normally, it is best to pair synergy materia with the materia of an element that most enemies are weak to, as this also often helps pressure the enemy. An otherwise particularly devastating combo is pairing synergy materia with Comet Materia. This will cause the user to cast Comet after the player-controlled character attacks. Because Comet has such a wide area, is such a strong attack that also knocks targets back and interrupts them, and is non-elemental, the ability to spam this for no MP cost can be incredibly effective against enemies. The main drawback is Comet's cast time, in which case an element-based materia may be better. Other non-elemental spells are Bio and Quake. However, even with elemental materia, the player need not fear their allies healing enemies with synergy: as long as the enemy's elemental absorption status is known in the Enemy Intel, AI-controlled party members with synergy materia will not use it if the enemy is immune to or healed from using the spell against them.
If synergy materia is paired with magic materia that are combinations of two magic materia, a random spell is used: e.i. with Fire and Ice Materia, the synergy-user may use Fire or Blizzard, and will never use the one the enemy resists, automatically favoring the one the enemy is weak to; as many enemies are weak to one or the other, and ice also builds stagger, this can be an excellent pairing. Poison and Petrify Materia may also be a good pairing with synergy, as the spells are non-elemental, do not have too long cast times, and inflict status ailments as well, and Quake commonly interrupts enemies by launching them off their feet. Thunder and Aero are good with synergy because they always hit, and Thunder has an especially quick animation while Aero can pull enemies.
It is possible also to pair two copies of that magic materia with synergy materia. For example, pairing synergy with a materia and then pairing another copy of that materia with HP Absorption Materia or MP Absorption Materia can heal the user cheaply at no cost, while pairing another copy with Magic Focus Materia can increase synergy's strengths further. This cannot be used with comet materia, however, due to comet materia's rarity. Magnify Materia does not affect synergy.
Aerith is a strong candidate for making use of synergy materia, particularly with comet, due to her 
Magic Attack. Both Tifa and Yuffie can build ATB rapidly, meaning a party with either (or both) of them and Aerith can allow them to build and use ATB often for Aerith to then follow up their frequent attacks with spells of her own. Using 
Haste and other ATB-regenerating methods also ensures synergy goes off rapidly.
If the synergy-user stands on Arcane Ward, the follow-up spell is cast twice, and if Yuffie uses synergy and has Doppelganger active, she also follows up twice, or four times if Yuffie stands on an Arcane Ward (however, abilities used by Doppelganger are 30% of the first use's damage, so it is not equal to Arcane Ward when used alone; follow-up spells used via synergy materia being at half potency also compounds with both Arcane Ward and Doppelganger). If Aerith has 
Auto-Unique Ability Materia equipped, she stands on her Arcane Ward more often even when AI-controlled as she keeps warping onto her wards automatically. If the synergy-user stands on Radiant Ward, they are immune to interruption and damage when casting. Both Arcane Ward and Radiant Ward can be overlapped, but if placed in different parts of the battlefield, Aerith can warp between them to stay away from enemies, and will do so automatically when not in the player's control if she has Auto-Unique Ability equipped.
The downside of using synergy is that the spellcasting character will have poor ATB-regeneration as they will be using their time with follow-up spells to the party leader's actions. If the character would be doing more damage with melee attacks or their other abilities than with half-powered tier 1 magic spells, using synergy on them is not worth it. This is why targeting an enemy's elemental weakness, helping both with pressure and damage, and using the wards, is so helpful for synergy users.