Traits

A demon will have 0 - 4 traits associated with their being, which define some passive bonus or detriment that will affect them during battle.

Acquisition
There are a number of ways a demon might acquire the traits they have:

Generation
When a demon is generated (i.e. they are spawned somewhere in the overworld), traits 1, 2, and 3 are randomly assigned depending on the type of demon. Every demon type has a number of traits, up to 8, which might be assigned given a certain percentage chance; the closer to 0 this trait is in the list, the sooner it will be considered, and once successfully rolled for, the generator moves to the next empty trait and tries to roll that one, until all possible traits have been tried. This may leave the demon with all 3 traits filled, and it may leave them with none.

Fusion
When a demon fuses with a different demon, their trait pool may also absorb the possible traits of the demon they fused with; these are also rolled for, to check if they will be absorbed or not. While this does not lead to traits explicitly, they do change what kind of traits the demon may have using the following methods of trait acquisition.

Reincarnation
When a demon reincarnates, their traits are rolled for again, potentially resulting in very different traits than usual if their trait pool has been modified.

Mind World
When a demon's mind world is explored, there's a chance that a Psychologist (one of the Dream Citizens) will appear, who will give you the opportunity to reassign one of the demon's first three traits with a randomly selected assortment of traits from the demon's possible trait pool. This can be handy for removing undesirable traits.

Possession
A demon soul will grant its 1st trait to the 4th slot of the possessed demon.