Elements and Races

Red Mage: The other meta class from NES is just as capable here, especially after promotion which shores up the one weakness of needing higher levels than other classes to get their spell charges. They can fulfill a lot of different roles at once: physical and magic damage, buffs and debuffs, even token healing. They're not quite as drag and drop as a Warrior, but they still fit nearly anywhere.
Full
Class Synergy Thoughts
WAR Great Whatever a Warrior wants, a Red Mage can do for them. Buff their offense, provide AoE, add defense with ARUB, even heal in battle.
Class Synergy Thoughts
RDM Good Double the AoE! Double the buffs! Redundancy on LIFE! What's there not to love? Your wallet might hate you if you want to spell both of them up, I guess.
WHM Good Redundancy on LIFE is always nice if one user goes down. White Mage can also help make sure undead die.
BLM Okay Double the offensive spells means double the pain. You might be able to get use out of BLUFF plus a status spell, but they're niche and it's kind of excessive with SABOTEUR.
BLU Okay See above, minus the point about BLUFF and with the extra consideration of not having to spend money on Blue Mage's spells.
DNC Good A Red Mage has enough offense to benefit properly from Dancer's buffs.
GEO Okay Geomancers can buff magic damage. It's so late it will barely matter (which is a problem with every other magic class except Evoker), but Red Mage can still benefit from the FORGED IN series in the meantime.
LNC Neutral Unlike most physical classes, Lancer won't benefit as much from FAST because JUMP isn't affected by it. They will on the down turns, but turn order is a problem.
TRA Good For whatever reason, FASTing a Trainer fasts their controlled monster. AoE also doesn't affect allied monsters.
EVO Good The problem with fighting Evoker bosses is that their charge attacks do way too much damage. A Red Mage can help against some of them by giving party-wide resistances. It's not too hard to make room for the spells either.
TIM Good A Red Mage can benefit from Time Wizard's DOUBLECAST. Or even FAST2 if needed.
GRM Okay Being another user of the anti-spells, Red Mage can provide ammunition for EXUVIATION. On the other hand, they can also just use an AoE spell themselves.
BRD Good Whether the buffs are physical or magical, a Red Mage can use them. FAST on harps is also interesting, even if TMPR won't work.
DRK Good Dark Knight appreciates having a FAST caster. You also get to double up on AoE with their BIO2, and have some insurance with LIFE if you want to abuse MISERY.
SPL Good There's two considerations here. First, Red Mage is physical enough to benefit from IMPART and can add FAST to the mix. Second, EXIT allows you to warp out with the ADAMANT and access the Spellblade's secret skill sooner.
ARC Okay FAST is nice for bows and helping an Archer get kills, as are their status spells. On the other hand, a Red Mage can just blow the enemy up with an AoE before the Archer can do much.
RON Good Giving a Ronin FAST is obvious, but has another advantage: it makes them generate TP faster.
MCH Good After class change, an Enhancer will make a Red Mage a better caster than a Black Mage, and make their status spells stupidly accurate.
Complex mixes - Red, Blue, Dark Knight. This will remove any temptation to waste a spell slot on DARK with Red, because Blue gets a spell that does it for free.

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