Can you tell me what Cultivation Masters is?

Had to find this one out for myself - Cultivation Masters are the 6 heroes you have leveled yourself, instead of being boosted to the level of the lowest level hero that isn't level 1.

You can only level 6 characters, the rest will automatically level up to the lowest level among those 6 characters. You need to use the reborn option (on the left-hand side when you select a hero) on the ones you have leveled up yourself until you have enough space among your 6 highest level heroes to manually level up new heroes.

Check which is the least character in the spirit level then upgrade that character then every character in the game also levels up