I almost never use fairies because of their massively reduced health. Bandit assassins eat fairy mages for breakfast. An extra 3 MAT isn't worth the chance of your mage having to recover from a severe wound.
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It's 8 mat. Starting is 15 so that's more than a 50% increase. With the starting staff it's still about 20%. There aren't many ways to increase mat so it's pretty significant. Fairies also get a 25 evasion bonus which more than balances out the hit point loss given how evasion works in this game. Starting enemies have about 90 hit so with rouge and thief fairies have a 45 evasion or 55% evade chance compared to a 20 evasion or 70% evade chance causing the fairy to take about 20% less damage than the elf while having 77% of the hp. 20% less damage is the equivalent of 25% more hp. This ratio will only favor the fairy more as the game goes on and evasion increases. A fully kited out fairy has more evasion than any monster's base hit rendering the question of hp largely moot. Really tho even in the early game the extra magic attack with the master's lust relief buff makes the difference between one-hitting most bandit den and goblin den enemies before they can go with entangle and taking a bunch of damage from half dead enemies which largely renders hp moot.
edit* Apparently I was only looking at elf rather than dark elf which is a lot better. The fairy is still the stronger caster as evasion is just better than having more hp, especially when you take things like healing and status aliments into account.
I'm still personally going to disagree with you. I've had a Fairy take one too many hits from Assassinate to have much of any sort of faith in evasion. Obviously I just kept getting hit with some pretty shitty RNG, but it happened enough that the extra 3 MAG a fairy offered just wasn't worth it, particularly since early game combat in this game can be fairly brutal before you've accumulated enough gear, stats, and combat oriented slaves.