Spirit Species

Spirits
Strictly speaking spirits are a separate category. Even though most of them work a lot like creatures, and the rest work a lot like humanoids there are subtle differences with all the other categories. (mainly in "species gene" inheritance behavior)

Spirits can be either summoned by MC or recruited from portals and some events.

The ones summoned by MC get genes based on MC's "native" (without equipment) magic stat, and it would seem that their max level gene is always 5 when summoned (while the rest is distributed randomly), their genes total can also be improved through MC's skill tree "spirit genes" upgrades (for evo and creation only, and NOT retroactively) and Elementalist MC profession.

Normally MC chooses an element at game start and can only summon spirits of that element. Initially only Fire, Water and Grass are available and the other four elements need to be unlocked (by leveling up a spirit of that element to level 10). MC of an Elementalist profession can summon spirits of all elements including the ones not yet unlocked as starters.

Only the basic spirit forms can be summoned, and a summoning typically costs 20 exhaustion. Exceptions thus far are stone (10 exh) and shadow (10 exh, 10 morale).

Summoned spirits are created at medium size (except stone spirit which is created small).

The wild ones have the usual gene generation, including max level genes and are recruited the same way as any other character. Their size can also vary. (you can even find an enormous stone spirit)

Fusing
Spirits can fuse with any other character (so long as that other character is eligible to fuse at all) except with another spirit. To fuse two spirits together at least one of them must have a "catalyst" genetic trait.

Spirits normally do not pass on any species genes when fusing, but DO pass them on when breeding (making the resulting character impure).

Evolution
Spirits have an evolution system similar, yet superior, to that of Druid's seedlings.

Any spirit still eligible for evolution will have an "evolving" trait which doubles (multiplicative effect with other exp increasing genetic traits) it's exp gain.

A spirit needs to be at it's max level in order to evolve. Evolving always resets the stats and level of the spirit, permanent consumable use count also resets (but the genes added by SoulReaper's special consumable do not). Genes and genetic traits are kept.

All base forms with max level still below 20 can be reshaped, increasing their max level gene by 3 and randomly increasing other genes by a total of 2 (in-game description is incorrect here, it is possible for only ONE gene to increase, but it will increase by 2 in this case). Specials are kept when reshaping.

All base forms and some non-base forms can also be evolved into next form. Spirits keep some skills of previous forms when evolving, Specials are changed when changing form, including the "adapted" special which is lost (while the effect of adaptation remains). Evolution into mount forms is accompanied by one tier of size increase.

Unlike seedlings you can choose level type and a learned skill upon a form changing evolution (including evolution into base species).

Thus far all the evolution-eligible forms are creatures (non-humanoids) and spirits.

Some of the final forms are humanoids, and some are no longer spirits (so far all the base species and the ring beast).