In Nioh, ki is your stamina. You need ki to attack, block and evade. Every attack, block and evasion consumes a portion of the green ki meter in the upper left of your screen. Fighting and winning in Nioh is largely dependent on managing your ki.
When you're out of stamina, you can't swing your weapon or evade attacks. If you run out of ki, you'll stand exhausted, huffing and puffing and unable to move or defend against attacks.
Ki regenerates over time. Swing your weapon or sprint, and you'll lose ki. Wait a few seconds, and you'll regain what you lost.
You can regain ki rapidly with a ki pulse. As you're attacking, blue orbs will gather around your body and will coalesce around you as your attacks conclude. Press R1 at the moment those orbs enter your body, and you'll execute a ki pulse.
A ki pulse immediately restores a chunk of your lost ki and speeds up the recovery of the rest of your ki meter. A perfectly timed ki pulse will completely restore your ki.
Ki and the yokai realm
Nioh’s demons (yokai in Nioh-speak) can create a circle of black smoke called a yokai realm. (They kind of look like a transmutation circle from Fullmetal Alchemist.) When you're in the yokai realm, your ability to regenerate ki is greatly diminished.
Keep your distance to keep your ki. You can usually lure a demon out of the yokai realm they created to avoid this effect.
A ki pulse inside of a yokai realm will disperse it (and still restore your ki at the same time). You can avoid yokai realms for a while, but demons will keep creating them. The longer a fight goes on — and it can be a long time for a boss fight — it starts to get crowded, and they’re hard to avoid. At some point, you’ll need to learn to disperse them — even while fighting for your life.
Ki and your equipment weight
If you’ve played a Dark Souls game (or D&D with a strict DM), you know about encumbrance rules. William is not a pack mule. The heavier his armor is, the slower he moves. Higher equipment weight rates also drain your ki faster.
You can check your equipment weight rate at any time by checking on William’s status, and a popup with the same information will appear at the top of the screen whenever you’re swapping or equipping new items. You’ll also see the information during leveling up at a shrine.
As your equipment weight rate climbs, the color of the percentage changes. Green is good, red is bad. By the time you get to red, you may have the highest defensive multiplier you can cobble together, but your ki will drain too fast to allow you to fight.