Mother of Beasts
Leader of the Burning Stars
The Searing One
Peripheral Mother

Antagonist

Old

Female

Fur, like everyone else

Long, long hair trailing down to her feet

White

25 ft

im not doin the math on that

Tear marks down her cheeks
Eye bags galore

She's slender, gaunt, and lanky

Burning Star

Black

She wants to burn away all the Denizens of the world

She is immensely powerful, but remains in one spot to work on producing more Star Beasts

Quiet and introvert

She wants to burn the world and rebuild it to have a place for her family
She wants to have children of her own
She wants to be a good mother

As previously stated, she is currently immobile on a separate surface world where she produces the Beasts that threaten the world.
In order to produce these Beasts, she has to cry her Flaming Tears for a long time until they create a body of water big enough for the Beasts to crawl out of. This causes severe emotional damage to herself much to the dismay of her allies.
She's emotionally vulnerable and sensitive and cries quite easily.
That said, she's very easy to anger and could cause horrible damage to a whole island if she could (she would never hurt her allies, however)

She's an immaculate leader despite her emotional issues and an amazing speaker
Her charisma comes from her motivation to make a world for the Burning Stars to live in rather than dying off in the cosmos
She also has a soothing singing voice that calms down the rambunctious Twins

She likes to sing to herself and writes her own songs
She loves to dote on the Twins like they were her own
She gets a kick out of teasing Brutus
She's very supportive of whatever diabolical schemes her team comes up with, especially [name of one of the members that I still need to think of] who is very insecure about his ideas

Although she secludes herself in her realm, she is very grateful to her team for keeping her company and believing in her dreams. She loves her team like they are her family and genuinely wants to give them a home where they can feel accepted. She has a prominent wish to have actual children, not just mindless Beasts that do not process her love. At times of weakness, she'll try to treat the Beasts as her children and mother them. But usually, this results in them harming her, which causes her to kill them. This causes plenty of anguish in her heart. She has very real desires to be a mother.

As a Star, Aunt Mat is naturally imbued with magical energies from the moment of her creation:
Flaming Tears: the tears from her eyes are what creates the waters that produces the Star Beasts that threaten the world. This is actually an ability that she taught herself and did not have this at the moment of her creation.
La Agua de las Estrellas: The pool of water that surrounds her that she made from her Tears. The Beasts emerge from this pool as her mindless minions that seek to set the world alight.
Melodic Authority: Her singing voice, although natural, is also an ability that boosts her allies and weakens her enemies when activated.
Astral Pyromancy: An ability apparent in all Burning Stars, they can cast flames from their limbs that burn eternal and cannot be doused by water. They are white and purple in color and leave burn wounds that cannot be healed. Aunt Mat has the most powerful Pyromancy of the team.

She can't really use her magic since she has to stay in the other realm. She's a terrifying force of nature that can't be subdued by conventional means. You can attempt an appeal to her heart, but given her emotional sensitivities, that could be difficult.

All Burning Stars are "born" with a relative understanding of their abilities and don't need to learn how to use them, rather how to master and utilize it to its full potential.

Her Children/ Beasts

Leader of the Burning Stars and Producer of the Star Beast

In the beginning, she was nothing. She was a fading gleam of light that only knew the cold expanse of the torn blackness. It drifted without thought or feeling through the cosmos and sought after nothing. Then. All of a sudden, something flickered in its heart: want. It didn't know what it wanted, but it wanted it badly. It looked for that want by soaring through the stars and searching for what its heart craved. It kept on and on for no one knows how long until it began to slow. It kept going until it got slower. And then it kept going longer until it got even slower. Then it stopped. It was so tired. It couldn't going on. It closed its eyes to rest. The coldness of the lonely darkness enveloped it completely as the want it felt waned and faded.

Until she felt warm. A lovely embrace had made itself known to her and opened her eyes to a new face. The Scorching One had made himself appear before her. He was... small. Not tiny, but she was only aware of a world that was bigger than her. And here was another face as white as hers with eyes as dark as hers asking, "What the hell are you doing?"

He was her first friend. Somewhere in her heart felt fuller. More complete. But still wanting more. She gave herself an identity. She abandoned her old title and donned a new one. With her name, she traveled with The Scorching one far and wide beyond the cosmos to find a home.

Home.

That's it. A home. She wanted a place to belong. She wanted people to belong. Family. Home and family was her want. With this new resolve, she gathered herself more Burning Stars to seek her wish. First was the dull twins. Then was the cowardous fool. All changing and growing with her presence. They were going to find a home for themselves. Even if they have to ride across every star. Even if they have to search every galaxy. Even if they have to burn a hole into their path, they were going to find a place to belong. Together.

Her name, "Aunt Mat" doesn't actually mean she's an aunt. She used to be called just "Mat" by Brutus, until the Twins came along and started calling them and Aunt and Uncle respectively. The "Aunt" in her name is symbolic for her character creation as a whole.