Warnings for some alternate characterization and canon-typical transformations.
Daisuke and Riku had married in their early twenties and Riku had given birth to a baby boy. They had decided on the name Koushiro, which they'd both wanted as a break from the usual Dai- names of the Niwa family.
"I love him so much." Riku said as she held her son in her arms.
Daisuke nodded, smiling as he gripped Riku tenderly by her shoulder. "He'll grow up to be a fine Niwa."
Daisuke imagined all the things Koushiro would get up to during his time as Dark Mousy and laughed inside. Dark and Krad would fight again.
Koushiro looked around for Daisuke and Riku.
It was after school and he had wanted to be friends with Mirei Hikari. Mirei, however, had brushed him off, claiming it was dangerous for the Niwa boy to be in her presence. When Koushiro wanted to know why, she had simply ignored him and walked off. Unsure why his classmate was behaving like this, he went home to look for his parents and get an explanation out of them.
"Dad? What is wrong with Mirei?" Koushiro asked of Daisuke Niwa.
Daisuke glanced up at his son from reading the newspaper. "Ah, Satoshi's daughter. She is a Hikari, Koushiro. Every generation of Hikari have a magical being live inside them that dated back to the first Hikari. We Niwa are in a similar position. She was likely warning you of Krad."
"What will happen to me?" The fourteen-year-old asked his father.
Daisuke put the newspaper down and shared a knowing glance with Riku, who had just come into the dining room. "You'll see soon enough." He smirked, remembering what had happened to him on his fourteenth birthday. "Why don't you head up and study a bit? Dinner will be a while."
"Do I have to?" Koushiro complained.
Daisuke laughed. "Yes. Sorry, son. It's a time thing, that's all I can say. You can come back down when it's dark." He couldn't wait to see the expression on his son's face after the transformation occurred. It would be so good to see Dark again.
Koushiro muttered angrily under his breath as he went up to his room.
Still in the dining room, Riku glanced at her husband. "Did you have to tease him like that? He'll be just as alarmed as you were back then."
Daisuke smiled, but knew Riku had a point. "I want to see how quickly he recovers from the ordeal. He should be able to brush it off with little trouble." He knew the first transformation was always the hardest; always was for the Niwa males.
Koushiro was feeling upset due to what Daisuke had said to him. "What does he mean by that?" Koushiro muttered to himself in a semi-mad tone of voice.
The teen wasn't sure what was going on, but he studied even though he was reluctant to. Once night fell, searing pain shot through his body and he staggered away from his desk. Groaning in pain, he clutched his head, before collapsing to his knees as his whole body shone with light.
"What is happening?!" Koushiro cried in surprise and shock.
The light eventually faded and a new person was in Koushiro's place. He stood up and looked at himself in Koushiro's mirror. "It's good to be back." He walked down the stairs and saw Daisuke straight away. "Yo, Daisuke!"
Daisuke turned and saw the taller individual at the base of the stairs. He waved and called for Riku. "Dark. It's been a while."
"He's taken a back seat at the moment. Too surprised to realize Dark Mousy is part of the Niwa family and has been since the first Niwa." Dark explained at Daisuke's mildly curious expression.
Where am I? Koushiro wondered from inside his own head. That light... A transformation? Focusing on the outside world, he heard the other individual mention Dark Mousy and he started putting things together. This must be the Niwa curse. The Hikari have Krad and the Niwa have Dark.
Riku came back into the dining room and stared at Dark Mousy. "Hello, Dark." Her smile was faint as she looked upon the one who had crushed her sister's dream. She remembered the task Daisuke had given her and held out a flyer, bringing Dark's attention to it. "We've already notified this museum that Dark Mousy has returned and intends to steal this Hikari artwork."
"Which one is it?" Dark answered, studying the paper flyer to remember the museum's location.
Riku handed the thief a photo. "A statuette they made a long time ago. It's cursed enough people, so you will have to seal the magic."
Dark nodded and got With to transform into his wings as he ran out of the house. Ten minutes later, Dark saw the museum. Dark managed to slip into the museum undetected and secured the statuette. But before he could slip away, Mirei was suddenly in front of him.
"I figured you would be Dark's new host, Koushiro." The girl spoke without emotion, as she stared at Dark. Or was it through Dark's being?
"Mirei, don't do this." Dark answered conversationally. "The boy likes you a lot. Don't be so distant."
The Hikari girl looked at the dark angel with a deadpan expression, as if she was asking: why not?
Dark was at a loss on this one. He'd known Satoshi the generation before. And he was sure Krad had never been this silent before. "Mirei..." He tried again, unsure of what her intentions really were. "At least let Koushiro talk to you. Don't keep brushing him off like you did today."
"Does he… really like me?" Was her response. Was she unsure herself?
Dark didn't know what was going on in her mind, but she didn't seem herself today. Of that he was sure. Were there problems between her and Satoshi? He had no idea. However… "Mirei, just be yourself and let Koushiro in. He won't hurt you. Trust me on that, okay?"