Natsumi here is a college freshman too! They could be friends and she could fill in as a (somewhat) bad influence, maybe? She's not a delinquent or anything but she's definitely about having fun and letting loose once in a while. You can't study all the time let's skip for a day and do something actually interesting kind of gal.

On the flipside, she's got pretty good morals and sense of what's right and wrong (pros of having a cop for a brother). So she could also be a good influence in some ways?