The first thing would be to simply choose your characters 'power level'. Be it at the start, or them going through the journey to -get- to that level. Personally preferring the latter, but..
Then would come your characters strengths and weaknesses. A Mage may be near unbeatable in a ranged fight (with being able to summon fireballs from the sky to bombard their opponent) but the moment you get close is the moment they may hesitate, or panic. A warrior with their Adrenaline-fueled rage-state would be the opposite. Unkillable in close quarters (unless against a Death Knight) but lacking in ranged.
You have to decide what your character is good at and what they suck at, as well as what they do to try and counteract their massive weaknesses. On average, a mage isn't conditioned to wear heavy armour, so they wouldn't be able to take much punishment before falling dead, or a ranger may eventually run out of arrows when hunting. It's all about optimizing your strengths and nullifying as many weaknesses as you can.
As for "Mana pool" it should (and usually does) work that way. -ANY- Arcane-based magic (So not Holy / Void) takes a toll on your physical body. Slight exhaustion causing headaches, nausea and dizziness. Continued exhaustion could lead to blackouts and even memory loss at advanced stages, given the strain it puts on your mind. This is why Mages (on average!) try to win a fight as quick and as effortless as possible. Because the more they throw at their opponent, the weaker they get.
At the end of the day, you just need to decide a FIXED set of where your character is. Not constantly fluctuate without justification. Fighting another roleplayer when you could use "Well, I could just 1-up their attacks and win" as a justification only makes you seem like an asshole. You dont always need to win a fight. Infact sometimes it's better to lose more than winning, as that gives your character IC motivation to GET BETTER. If you're constantly winning, then it's pointless to fight if you don't learn from the experience.
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