My Girlfriend Cheated on Me With a Senior, so I’m Cheating on Her With His Girlfriend

My Girlfriend is Having an Affair with Senpai (Part 1)

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When I, Yuu Isshiki, saw it for myself, my hands began trembling.

…No way… there’s no way, right…?

However, the notification that popped up on the screen of my phone cruelly told me everything—their relationship.

> (Kamokura-senpai) Karen, are you home yet?

> (Karen) Yup, I just arrived.

> (Kamokura-senpai) I just got home too.
But Karen, I already miss you.

> (Karen) Me too! I can still feel Senpai’s lingering touch!

> (Kamokura-senpai) [Good!] stamp.

> (Karen) [It was the best!] stamp.

> (Kamokura-senpai) Seriously, Karen, that was so good.
Why don’t you become mine?

> (Karen) Eh~ don’t you have a girlfriend already? Sakurajima-senpai, right?

> (Kamokura-senpai) But, you know, I think our bodies are really compatible.
Don’t you think so too?

> (Karen) I do!

> (Kamokura-senpai) Right? That’s why I want to keep having sex with you.

> (Karen) We do it at least once a week already.

> (Kamokura-senpai) I want to do it every day!

> (Karen) LOL.

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> (Kamokura-senpai) By the way, Karen, didn’t you say it felt better than with Isshiki?

…

I didn’t want to read any further than that.
I couldn’t.

I won’t scroll down.

After quickly taking a picture of the messages, I noticed several similar conversations.

The dates—definitely all days I hadn’t seen her on.

Apparently, Karen was “hanging out with friends”, “going out to dinner with family”, “finishing a report tomorrow,” and so on.

Karen Mitsumoto—my girlfriend—would, on those days, meet Tetsuya Kamokura, an upperclassman in my circle.

…They went to love hotels, and other such places.

According to online relationship advice for girls, it is the boyfriend, not the girl, who was at fault.

—It’s the boyfriend’s fault for not trusting her—

—The boyfriend doesn’t care for her enough, so she cheats on him; in other words, he deserves to get cucked—

—It’s not allowed to look through someone else’s phone without permission; they’re the lowest!— 

I put Karen’s phone back into the jacket it was in.

Karen had recently changed her phone’s security.

Her fingerprint authentication hadn’t seem to be responding well, so she switched to using a PIN code.

I know the PIN code.

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It was her ex’s birthday.

I found out by pure chance.

When Karen’s phone vibrated, I took it back out, seeing “Yoko-senpai” displayed in the notification.

“Yoko-senpai” is my senpai at my part-time job.

You don’t know Karen, do you?

And when I opened the messaging app, the image on the screen was identical to what I had just read about.

It was a love hotel.

As of right now, Karen was taking a shower.

I put on the clothes I had taken off and left the hotel we were in.

On the way out, I typed—

[I’ve got something to do.
I’m leaving.]

Instead of heading direct