Stamina
Not so much a "Hidden stat" as everything you do will revolve around this stat, though it is directly intertwined with your Endurance. For every point placed into Endurance, your Stamina will increase by 1. Your Stamina is not effected by any sort of buffs to your Endurance and will be based off your base Endurance. Your stamina may not increase past its maximum even if you use a transformation.
Once you fall below half your stamina pool, you will start to feel the effects on your body, reducing each of your stats by 1 level (this does not effect your maximum stamina pool and the lowest it can be is 1). This increases to 3 levels when you get below 1/4th of your stamina pool, and if you hit 0 you will pass out.
You may increase your stamina by either entering a release state, giving you the attributed stamina regain (written within the talent, and not to go over your maximum pool) or by taking a rest and not fighting for 2 consecutive rounds, in which case you will regain 2 stamina (not to go over your maximum pool).
Stamina Drain
If a Skill would directly drain stamina from an opponent, it will be at no lower than a Tier 3 Skill unless through certain Talent trees. This will drain 3 stamina from the opponent if conditions are met to do so, and will give you 2 stamina back. Causing net loss to be 2 stamina for yourself, while your opponent loses 3. If you fail in your attempt you still spend the full stamina cost and do not regain the 2.
Stamina Gain
There are two primary ways to regain stamina, transformations and passive regen. Neither allows you to go above your maximum cap. The transformation will only provide stamina regen for the first time going into that specific transformation, meaning that if you go into Shikai and regen stamina but seal it for whatever reason, rereleasing it in the same fight will not regen you stamina.
For your passive regen, this is from not fighting for 2 consecutive rounds of posting, gaining you 2 stamina not to exceed your cap. These rounds must be you not fighting at all, meaning no dodging, no throwing a punch, nothing. It is a short rest.