She looks exactly like her.

I know a teen male STEM and Chess nerd who is a genius. He is an outcast in a US public highschool.

She has a super sexy beautiful girlfriend who looks exactly like the school's most popular girl such as when it comes to height, physique, face, etc. The nerd's girlfriend was already his girlfriend even before the two of them are highschoolers.

The nerd's girlfriend goes to the same highschool as him. His girlfriend is introverted while the school's most popular girl is an extroverted cheerleader. The nerd's girlfriend is a doppleganger to the cheerleader. The two girls are not related by blood but looks exactly the same such as in height, physique, face, etc.

So how will he be?