The song in question is, of course, Gary Puckett's "Young Girl". I have taken minor liberties with the wording in order to accommodate the situation. Naru and Motoko jealously provide the voice of Shinobu's inner doubts, as Keitaro wrestles with a love he considers inappropriate.
Quietly, the door slides open to show us Shinobu, carrying a tray of tea to her beloved sempai. As she approaches, Keitaro fligns his pencil down in frustration. Not noticing her, he starts to sing...
Keitaro: Young girl!
Shinobu freezes, a look of worry on her face.
Keitaro: Get out of my mind! My love for you is way out of line!
Naru: (sticking her head around the door) Better run, girl!
Motoko: (next to Naru, severly) You're much too young, girl!
Mutsumi dances dizzily along the corridor, providing the
backing vocals.
Mutsumi: Oh, oh, oh...
Now we see a shot of Kitsune at her dressing table. In the
mirror, we see a reflection of Shinobu behind her.
Kitsune: (wistfully) With all the charms of a woman,
You've kept the secret of your youth.
Naru: (spins Shinobu around to face her, severely)
Don't lead him to believe you're old enough to
give him love - because it hurts to know the truth!
Mutsumi: Oh, oh....
Naru & Motoko: Young girl!
Keitaro: Get out of my mind! My love for you is way out of line!
Naru: Better run, girl!
Motoko: You're much too young, girl!
Next we see Shinobu trying out the lipstick she was given by Kitsune.
Naru: Beneath your perfume, and makeup
Motoko: You're just a baby in disguise!
Sharp scene change to show Keitaro turn and see Shinbou there.
Keitaro: And though you know that it's wrong to be alone with me...
Shinobu puts down the tray and looks at him...
Keitaro: That come-on look is in your eyes!
Mutsumi: Oh, oh oh....
Naru & Motoko: Young girl!
Keitaro: Get out of my mind! My love for you is way out of line!
Naru: Better run, girl!
Motoko: You're much too young, girl!
Naru: So hurry home, to your mother...
Motoko: I'm sure she wonders where you are...
Shinobu ignores them, her gaze fixed on Keitaro.
Keitaro slowly backs away from Shinobu.
Keitaro: Get out of here! Before I have the time to change my mind -
Shinobu slowly walks towards him...
Keitaro: 'Cause I'm afraid we'll go to far!
Shinobu walks faster... and then runs forwards into Keitaro's arms.
Mutsumi: Oh, oh oh, young girl..... oh oh oh young girl....
They cling tightly to each other, kissing passionately. Naru and Motoko look
on, shocked and green with jealousy, as Kitsune and Mutsumi look wistful.
Then the alarm sounded, and she got up to start making breakfast for the residents of Hinatasou.
Although Shinobu is not my favoured match for Keitaro (I prefer Mutsumi in that role) she is still my favourite character. The song neatly shows her feelings about her love for Keitaro - her knowledge that the age gap is too large, and yet her dream underneath that she can still have his love. She knows her love is hopeless, but wants to enjoy it while she can - Keitaro is a good person, and would not hurt her or make her lose her faith in love. In this way, she is lucky to have Keitaro as her first love, even when it is doomed to be unrequited. She also provides a good example to Naru, and shows what real love is, not the violent S&M parody that Naru offers.