So why? Why things are how they are?
Nobody knows other than the cause of all of this, the hand in shadows manipulating everything.
But what about a small reveal? A small story, of the fire stored in the Frost-Flare quirk manipulator, our Hikoori.
Know that the only way to understand what lies deep inside of her soul, you have to see beyond the black and white of life.

Why does Hikoori use her fire, while Shoto does not?
It all began in a moonless night that has passed on her five years old self, just a few days after her getting burned by the being she have always considered the shell that protects her of the harshness of her small world.
'It's all because of my fire' She thought continuously, nonstop. She blamed her fire, Endeavor's blood that's pumping in her veins.
'If I did not have him inside of me, if I did not have Endeavor inside of me, mom would have still been here...' Those phrases span and span in her small mind, torturing it beyond it's years.

Not that she hated her father, no. She hated Endeavor. For her, Endeavor was the symbol of the symbol of false ambitions and empty harshness. Oh, she hate Endeavor so much. After what happened to he mother, she refused to use what he have granted her, the power of darkness that could not prove it's ability to build. She thought of it as her curse.

But life cannot be simple, can it? It has to pierce through people's souls and suck away their remaining will to hang on.
That night, she could not sleep, like in the previous ones. Instead, she spent all that time crying silently. She was loosing more and more hope everyday, but she did not show it. There was a big hole in her heart, a hole eating her away more and more every day.
She was sitting in the mansion's garden, under the sakura tree and mourning silently like she did every night. Just as she was there, she heard a lullaby; a lullaby she knows very well.
"Oka-san...?!" She jolted her head immediately the moment the sweet humming entered her ears. She looked around her and saw nothing.
"Oka-san...where...?"She asked loud enough for her only to hear.
The humming kept going on, but was gone a little bit farther into the house. She followed it, hopeful for once. Through the rooms, the humming kept moving and moving, and Hikoori followed it tirelessly, like if she was following a mirage of water in the desert. After a turn beside her siblings rooms, her father's room, her twin's room, it stopped moving in front of a certain wooden shoji door, decorated with multitude of traditional Japanese flower patterns. Hikoori knew this shoji door very well. She froze in her place and could not open it nor move it any way.
She gulped, reaching her small fragile right hand to the window. But, it hanged mid air. She could not find the guts inside of her to open up, to see the place where... where there was no coming back. The screams she heard two years ago from this particular room, they have never died from her ears. She could still hear them, as if they were shouting out yesterday. She trembled and backed one step away. The lullaby, though, the lullaby was growing stronger and stronger, urging her to get inside. She had to choose between her mother or herself.
She, at the end, chose her mother.
She slid open the door. The first thing that met her sight was large, black shelf. It was decorated by Japanese trims all over. The shelf was opened, it's doors in the form of Japanese like windows. Inside laid a vase of white flowers, a mirror and two Japanese candle handlers. She approached the shelf warily, as the lullaby was the strongest at it's side. She sat in front of it, her head down in despair. She could not lift her head and look at the picture laid in front of the mirror, the picture of her brother Touya. Yes, this was the altar of her brother.
She sat there, staring at the floor, not daring to make any move. She could not feel the presence of her mother, yet she still heard the lullaby.

Suddenly though, the lullaby stopped. Strange dots of light surrounded her and floated around her in circles. The light was blue, but it was warm and loving, just like her lost brother's. She clutched her heart, feeling it was going to explode from the pain.
"Touya onii-san..." She muttered under her breath.
The dots formed a tall figure, his features not clear because of the blinding blue light. The figure reached a hand to her and patted her head. She felt a strange feeling of warmth and painful happiness wash over her.
The light knelt in front of her, his hand still on her head. She felt the aura of a loving smile, even though she could not see it.
"Do not hide it..." The light said. She blinked twice, waiting for further explanation.
"To save... help me... no-t hide... flame." The figure said again, his voice wavering and glitching.
She tensed. She wanted to protest, but the light placed a finger on her lips.
"No... time... love.. you" He said and disappeared in blue flames.
Then she opened her eyes, and found herself on her bed, in her room. Her body was warm, like if she was close to a gentle fire. In her hand, she found her lucky charm necklace she broke during her training with Endeavor two days before.
It was her father, he carried her from under the Sakura tree to her bed. She held the necklace closer to her heart, feeling that her father's love, that she was sure it was there, was stored in this necklace.
"Touya onii-san... did not leave... save onii-san... use flames..." She sleepily hummed to herself, and for once, after five days, she slept on her own will, the warmth finally engulfing the cold of distance and loneliness.