what does spirit do? I can't find it anywhere...

Also why does Quinna's regular attack do tons of dmg one attack then the next do barly any? in the same battle on the same monster even.

Spirit determines things like:

- How effective "good" status is in battle (ie. regen, float, etc.)
- How fast a character can recover from "bad" status in battle(ie. poison, blind, etc.)
- How fast the Trance gauge fills up and discharges


Quina's physical attacks are very randomized. Can't remember the formula and don't feel arsed to look it up right now, but it's so random that you're better off using Quina's blue magic if you want (more) constant results.

It affects Zidane's stealing rate too, though you probably won't notice because he sucks at stealing so much. :p

spirit also helps cahracters withstand elemental attacks ( i. e. fire, thunder, blizzard,etc...) i think. or is that magic evade/defense

Magic evade/defense would affect how characters withstand elemental attacks more than spirit would. That and wearing elemental-resistant/absorbant equipment.

ahhh...

Do I get a cookie now?

Also why does Quinna's regular attack do tons of dmg one attack then the next do barly any? in the same battle on the same monster even.
Base = Atk Pwr - Target's Defence
Bonus = 1 ... [(Lv + Str) / 8] + Str - 1
Damage = Base * Bonus