A Day Out with Big Sis

You awoke in your bedroom on a schoolday like any other. You jumped out of bed and threw open the blinds to your bedroom, confirming your suspicions.
It was a beautiful day today, and you couldn't wait to get out there and experience it!
Throwing open your window, you leaned out and shouted at the top of your lungs, "Good morning neighborhood! It's a beautiful day today!" You were given silence in return, most of your neighbors still being busy with getting ready for work or school or sleeping. You looked around and took in the sight of the houses surrounding yours. Most of them had their blinds closed, though a few had them open. One such house was the house of that one ponytail girl who wears those rainbow panties. She was doing her usual morning routine of what you assumed was a dry-skin rubdown, since she always rubbed herself down with a towel when she woke up. The funny thing was that she was always topless when she did it, even after you'd caught her attention a couple times by waving at her. She'd usually freak out and cover herself and shut her blinds, but she never learned to keep them shut after! So you kept waving at her to let her know that you could see her, and with how much you were doing it you hoped she wouldn't think of you as some kind of pervert.
Regardless, you had to get ready for school. You attended Hanzo National Academy, a huge but pretty normal school. The most unusual thing you'd ever say you'd ever seen there was this girl who had some kind of crazy eyes, like huge pink crosses in her pupils. You saw her hanging out with some white-haired girl with an eyepatch, who you later learned was named Yagyuu, and by the time you managed to flag them down you could only talk to Yagyuu. You thought that you two hit it off, despite Yagyuu being kind of stoic. Her attitude didn't matter to you, since you could be friends with everyone and anyone. You've always had this kind of attitude, and something about it just seemed to click with her, since she seemed to warm up to you really quickly. Now you wanted to hang out with her after school, and to your surprise she agreed to join you. Now you could barely wait to hang out with her today! So you threw your uniform on, grabbed your breakfast and scarfed it down, and ran out the door to get to school!

Some time after school, you ran into Yagyuu near the main gate.
"Hey Yagyuu! You ready to hang out?" you asked as she approached.
"Yes. Where would you like to go?"
"Uhmm... how about the shopping district!? There's bound to be something we could do there!" you said as you immediately started walking in its direction. "Come on! We got a whole rest of the day to hang out!" Yagyuu didn't say anything, but she started following you with her regular stoic expression anyways. Eventually the two of you reached the shopping district, and you both set about looking through stores for anything that interested either of you and engaging in small talk. You didn't see anything, but Yagyuu found some small squid and bunny figurines that she found cute and you helped split the cost for them.
After shopping, you both decided to sit down at a small cafe to relax, since it was starting to get late. You were both seated at a small table outdoors, and you were quickly served drinks not long after. "Man, there's a whole lot of nothing here, isn't there? Sorry Yagyuu."
"No, it's fine. We found these, thanks to your help," she said as she lifted the small bag with her figurines, as if to emphasize that you'd somehow done something.
"Nah, it's nothing major! Just saw 'em and wanted to show you 'em!" you said as Yagyuu placed her bag back on the ground, her expression unchanging. You both sipped at your drinks as something popped into your mind. "Hey Yagyuu, you know what you remind me of?"
"Hmm?"
"A big sister!" you exclaimed as Yagyuu's eye shrunk and a small stream of blood started flowing from her nose.
"A-a-a-a-a-a what?"
"A big sister! You're like a big sister to me! Big sis Yagyuu!" you exclaimed again as you gave her a smile. "You mind if I start calling you big sis?"
Yagyuu regained some of her composure as she stammered her answer, "S-sure. I don't mind."
"Alright! Well, I have to go, it's getting late and my parents want me home for dinner! See you at school tomorrow big sis!" you said as you sat up and sprinted off, after leaving her enough money to cover your drink.
Yagyuu just sat there for a little while after wiping the blood from her face, looking into her drink. "Big sis, huh...?"