Is there a proper way of setting custom data to a player/entity? Like setting a group name, K/D etc.
You could use Options from the permission API (I think), or you could make a custom DataManipulator to store the values as NBT in the player’s data
Doesn’t that involve modifying data that the player already has, or can I store new data on there? (custom dataManipulator)
They’re stored as new NBT afaik.