Tausai

The tausai is the most distinct of a person's souls and is considered to be a person's nature. 
While the first two souls are gained during infancy, a person doesn't get a tausai until puberty, though rarely is it known precisely when it arrives. One day, a child will catch a glimpse of an animal following them in their reflection in a pond and know that they have reached maturity.

Appearance

Each tausai takes the form of an animal, though only animals that can be found in the local region. They are intangible and usually invisible, though make an appearance during moments of intense emotion or whenever their human uses magic. They also appear during deep sleep, when it's thought that the person's panai is wandering through dreams. Certain magical spells can also make a tausai visible, and they can sometimes be glimpsed in the corner of one's eye at dawn or dusk, while half-asleep, or in reflections. When visible, they appear like creatures made of colourful light lurking somewhere in the vicinity of their person.
A tausai chooses a person whose nature aligns with their own. A bear tausai attaches to a person who is strong, protective, and ferocious. A raccoon tausai attaches to a person who is clever, curious, and sneaky. Sometimes the relationship between person and tausai is not obvious, but commenting on this is very rude and considered rather simple-minded. Although it may seem odd for a person you know as a coward and a cheat to have a moose tausai, there is clearly something in that person you missed.
Meanwhile adolescents sometimes have to go through a transition period of accepting their tausai. They may fear they are unworthy of a particularly impressive tausai like a bear or a wolf, or else be disappointed that they ended up with a spider or a rat.
The gender of a tausai always matches that of its chosen person, although because they are intangible and rarely visible, this is not immediately obvious except for animals with significant sexual dimorphism.

Colour

The colour of a tausai represents a person's deepest motivations. For example, a green tausai indicates its bearer is driven primarily by curiosity, while an orange tausai means the person's primary motivation is to make amends for something they feel guilty about. The colour frequently shifts over a person's life, especially after significant events. Fainter colours indicate a less strong drive, and pure white means the person has no particular motivation for life at all.

Nature

When present, a tausai strengthens their human and draws strength from them in turn. The burst of energy one gets in an emergency, a flash of quick thinking or an impulsive idea that turns out for the best, or the resolve to keep going in the face of adversity are all influence from the tausai.
They do not talk or communicate with the human they are attached to, as they do not have distinct minds of their own. Humans who do not regularly use magic may see their tausai only occasionally throughout their life, and not pay it any attention beyond knowing what animal it is and how that affects their sense of identity.
Only humans have tausai. Animals have a lua and panai, but never a tausai.

Magic

A tausai is essential for magic. They serve as a bridge to link the panai and lua to the spiritual world. Children who have not yet gained a tausai can study spells and practice their music in preparation for magic, but nothing can be done until they mature enough to receive a tausai.
If the person is significantly injured, either physically or mentally, the tausai leaves until they have recovered. Any magical ability they had in the past is inaccessible to them until they heal and once again have that connection to the spiritual world.
An important trait of a tausai is how it serves as a guardian for the other aspects of the soul. A tausai protects a person from magic that would otherwise harm their panai or lua.