A short while later, in Naoki's room on the second floor of the apartment.
Watching her younger sister sitting in front of the mirror, her face filled with tension, Rei sighed resignedly: "Hah, Merry really had to go and say something like that just now... Ayaka, you could've just refused."
"I can't."
Ayaka shook her head slightly. "It was Naoki who took care of me. It wouldn't be fair if my older sister repaid his kindness on my behalf. Besides, I might be staying here for a long time. I can't keep letting Naoki take care of me forever. So, becoming a maid... Well, think of it as a part-time job."
"A part-time job..." Rei smiled faintly. "Ayaka has really changed from how she used to be. Back then, Ayaka would never have said something like this. But... you're going to stay here for a long time? Don't you plan to return to that world, Ayaka?"
"At first I wanted to, but now I'm not so sure."
Ayaka paused in thought, then looked at her sister. "It's probably because I saw you... that I don't want to go back anymore. Eh, Sister... are you and Naoki..."
She didn't finish the sentence, but seeing the slight blush on her cheeks, Rei knew exactly what her younger sister was trying to ask.
Smiling, Rei picked up a hairbrush. "I'll talk while I style your hair. Let me brush it first. Look, it's completely messy."
"Because two hours ago I was still on a plane."
As she brushed Ayaka's hair, Rei began telling her about her relationship with Naoki.
"My relationship with the Master—no, with Naoki—is just as you're thinking, Ayaka. To outsiders, we could be considered a couple. I love him, and he loves me in his own ways. Though at first, I didn't follow him because I loved him."
"Eh? At first?" Ayaka looked slightly confused. "Sister, did you... only start loving Naoki recently?"
Rei nodded. "It happened after I truly interacted with him. When I first entered this body and met him, I only intended to repay a debt. Not just because he helped you a lot, but because he helped me too."
Six years ago, in that vast underground cave, Rei—who always understood Ayaka in her own way—was directly rejected by her younger sister for the first time. She truly loved Ayaka, but her love was too heavy, too intense, making her unable to understand Ayaka's feelings. She never truly cared about her sister's thoughts, only doing what she believed was right.
If not for Naoki's intervention and support, Ayaka would never have defied her sister's will for the first time—and afterward, a serious mistake might have occurred.
In that sense, Rei was helped by Naoki, even if it was help she herself barely acknowledged.
"Also, the body I'm using now originally belonged to him. I essentially took it over. That's why I willingly became his follower and helped him. But not long after, I started to fall for him."
The trigger was the Holy Grail War two years ago.
On the second day after the Holy Grail War began, due to the schemes of Merry and Morgan, Naoki was brought to the brink of death. The Heroic Spirit's power sustaining him was taken away, his mana was critically depleted, and his body was so weak he couldn't move at all.
When Rei learned this, she felt not only furious, but also thought Naoki might die there.
All the other Holy Grail War participants at the time were high-tier or S-rank Heroic Spirits. Not to mention Grand-rank Heroic Spirits like Solomon. Naoki, having lost nearly all his power, had no chance of surviving such a situation.
That's what Rei truly believed back then. After all, if it weren't for Arcueid's power keeping Naoki alive, he would have died.
But to her surprise, Naoki—who shouldn't have been able to move even a single finger—stood up again and defeated his former friend who had gone berserk, using only his own strength.
"Even though he was betrayed by the person he trusted most, even though his enemy was his most important friend, he accepted it all in his weakest state and overcame it through his own power."
The strength Scathach gave Naoki was merely enough to let him move again. It didn't enhance his combat ability at all. At best, it restored him to the level of an ordinary human.
Yet, in the end, he won.
Because they were the same person, Rei could feel Manaka's feelings toward Arthur. And when she saw Naoki directly defeat Arthur through Magecraft, Rei merged the images of Arthur and Naoki in her mind.
No—she replaced them.
She replaced [Sajyou Manaka] with [Sajyou Rei], and [Arthur Pendragon] with [Natsukawa Naoki].
"That's when I, Sister, began to love Naoki."
"Yes. After that, my feelings spiraled out of control. Using 'replenishing mana' as an excuse, I asked for kisses from him, and engaged in intimate physical contact with him on the sofa... Since he's quite slow in some ways, he didn't notice anything strange about me at all."
Remembering how Naoki had blindly trusted every word and action she took back then, Rei felt delighted. It was truly wonderful to have a master so naively trusting—it allowed her to advance their relationship without him even realizing it.
"Then, I worked a little over these past two years, developing our relationship into that of a lover and follower. Sometimes, we even play roles like this to experience new sensations—Alright, stand up. Time to change clothes."
"Okay."
Listening to her sister's story about the development of her relationship with Naoki, Ayaka began to notice a pattern. In their relationship's progression... was it always her sister who took the initiative?
"Sister, has Naoki never once taken the initiative with you?"
"In normal situations, no. It's hard to admit, but all his initiative basically goes to that succubus. But he never rejects my advances either—Ayaka, you should keep that in mind too, if you plan to make a move later."
"Eh? M-me?!"
Ayaka panicked immediately. "I-I can't. Besides, Naoki is your boyfriend... I don't even have feelings like that for him..."
"But you like him, right?" Rei ignored her sister's denial and tossed aside her shyness. "If you like him, you need to act fast. If you're too relaxed, someone random might just swoop in before you do."
"So, cheer up, Ayaka. If there's anything you don't understand, you can always ask Sister—there, that's good enough."
After fastening the final ribbon on Ayaka, Rei gave her sister's back a firm pat, as if giving her a push forward.