{1 year 4 months time skip}
I was sitting in my room, watching some anime when I suddenly felt someone's hands around my neck. I quickly turned around to see Serafall standing behind me with a mischievous grin on her face.
"How are you, Ren? Did you miss me?" She asked with a playful tone. "If so, you don't have to worry about that for some time. How about you come with me to the underworld? I want you to teach my So-tan about magic as you're the best with it. Do it for me, please."
I thought about it for a moment and decided that it could be a good opportunity to meet Serafall's family and the sister she often talked about. "Okay, sure. But how will your family feel about a random person teaching her? I'm not well-known, so…"
"Mou, you don't have to worry about anything. Your Sera has got everything covered. All the Satans are with you. Aju-chan likes your magic knowledge, Zechs-chan sees you as a rival and a friend, while the sleepyhead is grateful for the comfy pillow you gave him. Due to all that, it was easy to make you an Ultimate-class devil. The devil's council can't do much since all of us agree that having you with us is the best thing. When you come with me, you'll get your evil piece too." Serafall explained with a grin on her face. And yes, I had told her about my race.
I shrugged and said, "Okay, let's go then." I quickly changed into something nice but not too grand and followed Serafall as she teleported us to the train going to the underworld.
During the train ride, I couldn't help but feel a little nervous about the upcoming Evil piece ceremony. "So how will this Evil piece ceremony go, since my circumstance is not normal?" I asked curiously.
"Well, it'll be anonymous. Some high-ranking people will know of this, but no one other than the devils will know about it." Serafall reassured me.
"Okay, then why don't we enjoy this for the time being," I said with a smile.
Serafall's eyes lit up, and she grabbed my hand, leading me to a private cabin on the train. We spent the rest of the journey in each other's arms, exploring each other's bodies.
After we arrived in the underworld, we cleaned ourselves with magic and made our way to the Sitri area. As we approached a castle, I saw a man, a woman, and a young girl waiting for us.
"Greetings, my name is Ren Nakamura. You must be Mrs. and Mr. Sitri, and you must be So-tan," I introduced myself casually.
Mrs. Sitri looked at me with a warm smile and said, "Yes, we have heard a lot about you from Serafall. Thank you for coming to teach our daughter."
I nodded and replied, "It's my pleasure. I'll do my best to teach her everything I know about magic."
So-tan was a little shy, but she was curious about me, and we hit it off right away. Serafall and I spent a few days with the Sitri family, and I got to know them better. They were kind people, and I felt grateful to have met them.
The Evil piece ceremony went smoothly, and I received my evil piece without any problems. Serafall and I returned to our daily lives, but something had changed between us. We had grown closer, and I felt more comfortable with her. I knew that I could trust her with anything, and that made me feel grateful.
"Don't call me So-tan! You may call me Sona, but never So-tan." She says this while muttering something about 'their multiplying!'.
"Sona, that's not the way to talk to our guests and your new teacher." Mrs Sitri reprimed her daughter.
"You have got to be kidding me, nee-sama said that he is only 14 years old like me, and she never said anything about him teaching me." Sona says with disbelief.
Sera explained about our situation. "Well that's because I wanted it to be a surprise, So-tan. Plus he's much stronger than me, he even beat Zechs-chan in a fight. Aju-chan even respects him for his knowledge in magic so I think he'll be a great teacher for you, he even helped me get stronger than before. Keep these things a secret for now, okay."
Sona couldn't believe what she heard. 'Even Beelzebub-sama respects him, then he must be very smart, I'll challenge him to a game of chess later.'
Sera looked at Sona's face and said. "Oh no you don't, you are not allowed to have a chess match with him. I won't allow it."
"Ee, nee-sama, does that mean?..." Sona looked at me and thought. 'Is this the person my sister's loves? He's about my age, but since she won't allow me to play chess with him means he's my future brother-in-law? Or is he a total scumbag who my sister won't let me be with. Much likely the first one.' Sona intripated.
After explaining our relationship to them they quickly gave us their blessings as Sera's mother wants a grandchild quickly as she's getting jealous of Venelana Gremory getting a grandchild. As Grayfia gave birth a few years ago.
I'm teaching Sona about magic control and how it's important for water magic. We continue with control and shape manipulation of water for the first week. By this time I already got my Evil Piece and got crowned as a Special class devil as I'm special. I've met many people and I helped them in some ways so most of them are grateful to me and respect me a lot due to my strength.
Currently I'm teaching Sona and her queen how to use other types of basic bagic, while Serafall is next to me, with her hugging my arm.
"Sensei, can I take a break today? I want to meet my friend today." Sona asked.
Serafall teases Sona first. "Moo, So-tan I told you to call Ren-chan Nii-sama. But yes it'll be good to meet Rias-chan and the others. I'll ask Fia-chan to inform Venelana about this."
So we now go to Gremory territory and when I get there I see 3 figures. Venelana Gremory, Grayfia Lucifuge and Rias Gremory.
Grayfia greeted us first. "Good morning Ren-sama, Sona-sama."
"Hello it's good to see you again. I'm Venelana Gremory." She says with her milf aura. We met a few times when I was with Zechs discussing cute pics he took of Rias.
"Nice to meet you Ren-sama and also Sona." Says Rias with respect, looks like she was informed about me unlike with Sona.
"Nice to meet you too, I've heard a lot about you from you brother."
"Hmm, what did Nii-sama say?" She asked.
"We mostly talk about your progress but now me Sera and Zechs get pics of you and sona, and discuss which is cuter and best." I say as I took out a few pics of her childhood.
"Give that back." Rias jumped at the picture as I dodged it. She completely forgot her status as an heir of the Gremory household as she heard it.
Sona muttered something with a smirk; 'Now you also have to go through the same as me.'