Your character in Nioh 2 has a Yoki Guardian spirit tied to their soul. This allows you to temporarily transform into a powerful being capable of doing a lot of damage and taking on a demon-like form. You choose what your demon looks like at the start of the game, but it varies depending on the type of Guardian you choose to wield. But you cannot transform yourself into it freely. To do this, you must meet specific criteria.
At the top left of your screen, you have your health, ki, and Amina bars. At the base of these three bars is a circle, and most of the time it lights up, showing the outlines of the Guardian Spirit bound to your character. While the outline of this guardian's spirits is fully highlighted, you can summon your Yokai form for a short time. To become your Yokai shape, you have to press the triangle and circle buttons at the same time. After using it, the full circle disappears and you have to put it back together.
To replenish your Yokai meter, you need to engage enemies in combat and perform melee attacks against them. You can do this by naturally playing the game, battling baddies as you explore an area, and then summoning your Yokai Spirit whenever you need a little extra strength to take down a particular enemy. Most of the time you want to save it for mini bosses, like Ippon-Datara or Enki, and bigger bosses like Enenra or Yatsu-no-Kami. If you die against a boss and your Guardian Spirit remains in its arena, you must choose to summon it from a shrine or immediately return to combat to try again.
Mastering your transformation of the Yokai mind is important because it can quickly turn the tide of combat in your favor. If you don't look to see how full it is, you might end up over your head during a battle. Make sure to regularly sweep the areas you explore to engage enemies in combat so that it is constantly recharged.
