The three elements that compose a human: Mind, Soul, and Body.
In EXTELLA, in order to escape from Titan Altera, the Main Character split himself/herself into these three elements, splitting the Regalia at the same time.
Below is an explanation of each element’s key qualities.

The Mind:
Master of Nero. His or her personality is based on personal experiences and passion.
The Mind is optimistic and outgoing, though not particularly thoughtful.
Even after being informed that he or she will disappear before long, the Mind keeps up a charming smile, believing that it will all work out somehow.
Coupled with that charm is a decent sense of morality, rationality, and justice.

The Soul:
Master of Tamamo no Mae. [Unlike the Mind, whose personality is based on life experience,] the Soul acts more on his or her core nature.
The Soul is optimistic, but rather passive, lacking in motivation and decisiveness.
Ephemeral and a little timid, the Main Character ends up being pushed around by Tamamo, who grows into something of a villainess after acquiring the Regalia.
In contrast with the Mind’s wild passion, the Soul exudes a sort of cool sensuality that enraptures even Tamamo by presence alone.
He or she possesses the philosophical nature and acumen of a transcendent being who is unmoved by emotion.
Although he or she may seem fragile at first glance, the Soul can achieve a state of surprising clarity when faced with a decision.

The Body:
Master of Altera. Driven by instinct, he or she has an open personality that persists even on its own, without the presence of the Mind or Soul.
As well, without those two essential pieces, and despite being born from the same “root,” the Body has a stronger sense of physicality.
In a sense, the Body is in a unique, virtually blank state, which means he or she can naturally and impartially assess Altera’s character.
And so, even after being imprisoned by the Titan, the Body interacts her not as an enemy, but simply as a conversation partner. On those terms, the two begin to understand each other. Although in truth, for the Main Character, it’s love at first sight. The Body has no memories, and thus the default state is to continuously observe everything in sight.
Without memories, the Body is constantly pondering his or her identity, the unnatural state of his or her existence, and, by extension, whether he or she will eventually disappear.
But instead of being fearful, the Body is simply inquisitive, just like any newborn intelligent life form. His or her tendency to overthink is born of a single-minded desire to know who he or she really is.
Later, the Body learns, and comes to accept, that his or her fate is to disappear no matter what happens. With that mystery solved, he or she then focuses on releasing Altera from the cruel fate that awaits her.
In all other respects, the Body’s base personality is the same as the Mind.
The Body is both pessimistic and bold. At first, the Main Character is headstrong (tsun), but they show their weakness once they reach a certain point (dere). So they are tsundere.
The Extella series is made so that there is almost no difference between the male and female Main Characters, but the Body is an exception, and has clear differences.
The relationship between Titan Altera and the female Main Character is one of a fearsome alien deity and the pure holy maiden who worships her.
The reason we had the female Main Character’s new costume designed as a dress was purely so that we could emphasize that imagery.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA