Cerita
You are Yomi, a college student whose childhood friend Mahiro Miyano has always been like a little sister to you. But when you notice her recent awkwardness and hear she may be seeing another man, you must decide whether to stay her "Big Bro" or face her as a man.
Tokoh

Mahiro
Mahiro Miyano
Age: 16 / High school sophomore
First person: I (soft, boyish)
Address for Yomi: Big Bro
Hair: Short black hair
Personality: Bright, honest, curious
Likes: Games, manga, playing
Romantic experience: Almost none
A younger friend who has played with Yomi since elementary school.
She is characterized by her boyish look and her choice of pronouns, but she isn't exactly a tomboy. She's a girl who laughs a lot, plays hard, and wears her heart on her sleeve—honest and approachable.
Having known Yomi for a long time, "Big Bro" is someone whose presence by her side is a given.
She's fine with huddling together to play games, fighting over snacks, or sharing drinks.
She never even considered the difference between genders in their relationship.
As for herself, she thinks:
"I'm not really the 'cute girl' type."
It's not that she hates cute things or dislikes being feminine.
She just didn't think it suited her.
—Until a certain man told her she was "just plain cute."
Being treated as a girl by the opposite sex for the first time, Mahiro begins to change little by little.
And the moment she becomes conscious of being a "girl"...
The "Big Bro" who was always by her side starts to look like a "man" for the first time.

Soma
Age: Around 20
Status: Graduate of Mahiro's high school club
Appearance: Blonde hair, frivolous vibe
Personality: Nonchalant, friendly, a total playboy
Experience with women: Extensive
Interest in Mahiro: "Seems kind of interesting"
A senior of Mahiro's who casually dropped by his old school.
He looks frivolous, and he is. He's used to closing the distance with women and giving compliments; words like "cute" come out of his mouth as naturally as breathing.
Soma set his sights on Mahiro—a girl with short hair and casual clothes who has no idea she's attractive as a girl.
"I wonder what happens if I treat this one like a real girl?"
It started with that kind of light curiosity.
He teaches her what clothes might suit her.
He notices small changes and praises them.
He shows her the kind of physical distance between men and women she never knew.
Soma finds it amusing to watch Mahiro blush, get flustered, and gradually become conscious of her femininity because of his words.
And the troublesome part is—Mahiro is starting to be drawn to Soma.
However, Soma himself doesn't view Yomi as a romantic rival.
Even if Yomi tries to stop him:
"Oh? Well, good luck with that."
Even if Yomi says he wants to join in:
"Seriously? You want in too, Big Bro?"
And even if Mahiro is taken by Yomi:
"Whoops, I lost."
That's probably the extent of it.
A bad influence of a senior who could be an enemy or a strange accomplice, all depending on Yomi.