Monsters in Kamisekai aren't strictly defined by a set of rules. As a GM, you'll mostly be leaning heavily on your GM shield to handle combat and moves. Remember; you are in control of the monsters, and can always adjust their strength mid-fight.

You can keep track of your monsters any way you want. You don't even have to write them down if you don't feel like it. But when planning a more complicated session or a longer campaign, it might be useful to have them for future reference. Here's an example template.

Enemy Name: 
HP: 
Armor: 
Weakness:
Strength: 
Moves:
  Attack - <die> damage.
  Attack All - <die> damage split between players
Spells:
  None

Now, let's fill it out! Name and HP are self-explanatory. Unlike players it can be a range or estimate rather than a fixed number.

While it's absolutely not necessary, consider giving your monster a particular set of strengths and weaknesses against their attackers. Perhaps a plant-like monster would be weak to fire, or a ghost would be weaker if exposed to bright light. Be creative!

Next, abilities. Monsters do not need to use the same abilities as players. They can have all kinds of creative, obnoxious skills, magic, or moves. They can inflict ailments not in the standard list of effects, they can put the players into exciting new kinds of peril, or strip their clothing, or even manipulate them directly. For the sake of your sanity, you'll probably want to keep the descriptions of the moves simple. This is to remind you how your monster works, not describe it to the player.

Likewise, you can pick out (or make up!) some spells for the creature to use. Not every monster needs to know magic, but it might not be a bad idea if they know how to cast a spell in case the players disable them somehow. Especially humanoid enemies.