If we were to be reborn, I'd like to be twins so we can play together.
Junko had always been more outgoing than Mukuro. Even though Junko felt nothing but despair her whole life, she made an effort to pretend. Mukuro wasn't one for socializing, going as far as to join a mercenary group at such a young age. She was more athletic and active than her younger twin, and served as a foil of her personality. There were a lot of things in her murky life Mukuro couldn't be too sure of- like maybe she wasn't totally in despair like her sister was. Maybe a seed of hope lay dormant somewhere. She would never know
But Mukuro knew one thing for sure: Junko was always right. Junko was more powerful than her, and higher than her. She was the tyrannical queen of an unwilling empire, with Mukuro as her faithful servant. She didn't mind. She wanted to follow Junko to whatever lengths she desired. The two were already in despair, death was a welcome friend. She didn't care about dying… right?
During her time at Hope's Peak, Mukuro had made friends with the classmates she would later go on to double-cross and cause the deaths of. Junko's orders were law to her, they must be followed. She knew nothing but despair her whole life. Until, however, she met that boy.
He smiled kindly at her, despite her cold personality. He always seemed to have happiness in his bright green eyes, and he always smiled. He'd even smiled at Junko, much to her disgust. That smile is what made her want him- and all of their classmates- to die horribly and suffer. And as the servant of Junko's evil, she had to follow orders. Junko's orders were Mukuro's law. So… why couldn't she stop crying? Why was she upset about it? Emotions were useless to her, thought dead within her, but they began to show their faces. Not even her sister could wake them, but someone so normal…
Did they call this love?
"Love" was a foreign concept to someone like her. She only knew despair, war, and Junko. But something broke through the dark barrier and flooded it with light. And much to her confusion, she let the light chase away the shadow.
The time had come to start the despair game, and Junko was being stubborn about carrying it out. Mukuro had to comply to her whims though. Junko's orders were her law, after all.
"Then, I'll masquerade as you. If someone kills me, you'll be safe. Their memories are gone; they can't tell the difference. And even if they remembered… we are twins after all. No one will notice."
Mukuro Ikusaba was not supposed to die. But Junko Enoshima changed her sister's fate, and sealed it. Her dearest sister had become soft, letting something as trivial as romance flood out her despair with such repulsive hope. Mukuro's final moments were spent being betrayed by the one closest to her. The despair of killing her own sister gave Junko twisted delight, especially while watching everyone reacting around it.
Mukuro Ikusaba died in place of her sister, whom she loved. And yet, she didn't resent her. The small seed of hope buried deep in her heart gave her that forgiveness, and in her final thoughts Mukuro wished to be Junko's twin again in another life.