This is just a small side story that happens before season 2. I hope you enjoy it.
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Y/N was currently in the Sitri domain and was holding a bag of clothes in both hands. The look on his face shows that he wasn't having the best time. Serafall was going shopping one day and asked Y/N to come along. With her bubbly personality, she was having fun. Y/N on the other hand...
Flashback
Y/N had went to his second home, which was the Sitri manor, and was visiting after the battle between Riser and Issei. The door had opened up to be his adopted mom, Serafall. Although Y/N wasn't really too into calling a teenage girl "mom", but as Y/N stayed around her he grew attached to the idea.
Y/N currently sat down in his old room that he lived in and he changed into clothes that were a bit more comfortable.
(A/N): The clothes that Y/N wears in the Underworld is the Swordcraft Academy outfit when Kirito was training under Sortiliena Serlut in season 3 of SAO.
Y/N then made his may to the common room where Serafall was sitting down on the couch. Y/N sat opposite of her and the two were drinking tea.
Serafall: So Y/N, how have you been doing?
Y/N: Fine I guess.
Serafall: Are you sure that was the path you wanted to take?
Y/N: I needed answers and this was the only path to take.
Serafall: (pouts) Mou! Why couldn't stay at home with mommy?
Y/N: Well I needed to see them again.
Serafall: Well...have you?
Y/N: Um...
Serafall: (taps foot) Y/N...
Y/N: Does watching them from afar count...?
Serafall: Do you even try to talk with them?
Y/N: I do talk to them.
Serafall: When you're not wearing the jacket and you don't have your swords?
Y/N: ...
Serafall: (sigh) You need to face them soon, you know?
Y/N: I know.
Serafall: Have you at least made more friends?
Y/N: Well...
Serafall: Aside from work.
Y/N: I--uhh...no.
Serafall: Do you have a girlfriend?
Y/N: No...?
Serafall: You need to get one! I want some grandchildren! You may not be my son by blood, but you're still my son!
Y/N: (sweat drops) Isn't it a bit too early for that though?
Serafall: ...I guess you are right, but I still want grandchildren!
Y/N: (sigh) Yes mom.
Serafall: Good. (stands up) Now then, it's been awhile since we last saw each other so I believe we need to have more bonding time!
Y/N: Please don't tell me...
Serafall: (claps hands together) Yep! Let's go!
Y/N: (thoughts) Oh boy, this is going to take forever.
Flashback End
Y/N was usually tired of his mother's antics. While he didn't deny her company, he didn't like the long shopping trips he had to go with her. As Y/N stood there, the people who lived in the Sitri domain gave him looks of pity. Y/N was recognized as the son of the Leviathan despite being a human. The people still respected him though as he helped some of the citizens while he still could.
Serafall then comes out of the clothing shop with another bag and hands Y/N the bag she was holding.
Y/N: Do you really need this much clothing, kaa-san?
Serafall: Of course I do! A girl needs to look pretty!
Y/N: You think so?
Serafall: Yeah!
Y/N: (sigh) This is why I don't care about what I wear...
Serafall: All you wear is black clothing!
Y/N: Because black clothing is the best!
Serafall: You need to wear something other than black.
Y/N: Well nothing really fits my taste so...
Serafall: Well, let us continue on to the next store!
Y/N smiles at his mother's enthusiastic self. The two were quite polar opposites. Y/N was an introvert and never really liked talking to people, but Serafall was always quite cheerful.
When they arrived at the next store, Y/N knew what this was. It was a cosplay store. Apparently, he had gotten some messages from Sona the other day and she had said the Serafall has recently wanted to be a "magical girl." A look of horror makes it's way to Y/N's face as said girl motions for him to come in with her. He took a deep breath and went in after his mother.
Of course, the whole store was filled with girls. Y/N was quite popular among the teenage ladies in the Sitri domain. The ladies were quite glad when they found out that Y/N was the adopted son of Serafall and not in a relationship together. Y/N stood rather awkward as all the eyes were on him and they were giving him flirty looks. Y/N was quite flustered and would turn the other way when the a girl would give him a flirty look. He heard his mother call to him and he turned his attention to the changing booth. He blushed a little as his mother did a little twirl and what not. The little cosplay had even come with a little plastic wand. Or at least that what Y/N thinks.
Serafall: So? What do you think?
Y/N: (blushes slightly) It looks...great...
Serafall: (grins mischievously) I see now...do you see you're mother that way?~
Y/N: N-No...! I-It's n-not that at a-all!
Serafall: (boops Y/N's nose) You're so easy to tease. (giggle)
Y/N: (sigh) Can we move on now?
Serafall: I've got one more to show you!
Y/N: Can you make it quick then?
Serafall goes back into the changing booth and Y/N still looked flustered. She was right though. Y/N was easy to tease as he never really talked to a lot of girls. It was a different story if it was a contract he needed to do. Other than that, if a girl tried to flirt with him, he'd either brush them off coldly or just get flustered and walk away.
Y/N was called once more and he looked back to the changing booth. The curtains open and he smiled a little. The outfit really did suit her.
Serafall: Well?
Y/N: It does suit you, mom.
Serafall: Really? Guess I'll buy these two then!
With those purchases out of the way, Y/N and Serafall ventured around the city, with said lady trying on various types of outfits for Y/N to look at. He couldn't really complain though. He enjoyed spending time with his mom. His real mother never really took him on to trips like these. She was either busy with work or she was a sleep as she always worked late at this office. The only one around was his father and his father did the best to keep his son happy. But that came to an end when Y/N's parents perished in a car crash.
After a few good hours, Y/N and Serafall returned back to the manor. It was time for Y/N to go. Y/N stood at the door and Serafall was saying her goodbyes.
Serafall: Do you really have to go?
Y/N: I still need to watch over them. Valper is still out there.
Serafall: Promise me you won't go down a dark path, my child.
Y/N: (hugs Serafall) I won't. Goodbye, mom. Stay safe.
Serafall: You too. If you have a girlfriend, let me know.~
Y/N: (chuckle) You'll be the first to know.
After their goodbyes, Y/N got on board the train that would take him back to the human realm. As he waited for his stop, he pulled out his phone and started scrolling through his pictures. He stopped at a particular one. It was his first day at the Sitri household. Serafall had both her hands wrapped around Y/N's waist and she pulled him back. She was about sixteen or seventeen here. Sona was next to him with Lord and Lady Sitri behind the two daughters. Y/N had a small frown on his face in the picture and was looking away.
Y/N: Should have been smiling in this picture. Oh well. You can't change the past now, can we?
Y/N smiles to himself as he puts his phone away and he looks out the window as the sun in the human realm was now rising over the little town of Kuoh.
Y/N: I'm home.