Spirits are neither humans nor monsters. They are simply souls who still wander the world. Most of them are invisible to a vast majority of creatures, and eventually fade away as their soul weakens. Once in a while, though, one will absorb the powers of the place they wander, and strengthen their link with the living world, giving them a somewhat physical appearance and the ability to interact with the world.
Being souls of the dead, spirits tend to have unsettling and even frightening appearances and voices.
Spirits cannot reproduce, even if they’re powerful enough to physically manifest.
Yokai are mythical creatures from Japanese mythology. They are incredibly diverse in appearances, powers, and origins.
(Note : most of those descriptions were inspired by those available on yokai.com, and adapted to better fit the lore of my personal universes. They may not be 100% accurate to real mythology.)
Kitsune are humanoid spirits with fox features. Generally known to be tricksters, they can shape-shift into both animal and human forms and use those abilities to pull various practical pranks, both harmless and dangerous.
Bakeneko are shape-shifting cats known for their mischievous and playful nature. They can disguise themselves as normal cats, humans, and even sometimes take the appearance of their masters. While not necessarily malicious, they tend to bring misfortune and might sometimes take the place of their master. Their tails are like torches, and they can also reanimate corpses and use them as puppets.
Nekomata are more powerful bakeneko, recognizable by their twin tails. Unlike bakeneko, they are often malicious. They start fires on purpose, and use their necromantic powers and influence to enslave humans.