I know. It doesn't make sense. 

Info about the MC:

Name: Deku

Age: 15

Relationship: Adopted younger sister of Deku by 2 years; likes Levi

Quirk: Ghost. Able to move through walls and door, but her body is left behind

so basically

Kyrenia's chilling with the boys when Diavolo calls

Turns out they have a guest exchange for a day

Guess who, dudes? The whole 1-A class!

Diavolo just decides to have an exchange thingy

Deku isn't his usual self; his sister's gone, so everything's really confusing right now

Kyrenia goes in like, "what's happening-" 

she spots Deku, Deku spots her

They stay in the same spot and just stares at each other

Ochako notices and asks Deku what's up

He doesn't respond and like instinct, they both run at each other

They share a hug while crying

She explains what happens and introduces them to the council members and they know what quirks are

Levi comes up, and they have eye contact

And you know, she's a shy nerd so she just starts stuttering "y-y-y-yo!"

Deku looks plain confused; then he gets it and gives her a big bro smile "Owo..."

Now they boys surround her; "Don't you have a quirk?" 

She nods,  then shows her quirk by asking Deku to catch her body

You can see her ghost form as she flies through walls

then the tour starts 

the rest is up to you!