Everything in this AU remains the same, EXCEPT the fact that people who would work well together as partners can be tied to one another. However, here's the catch! Your red string won't appear until the feelings are accepted by both people.

(To make it easier to understand I'll give an example)

Say that your OC has a crush on Kirishima (example-), for the red string to appear, Kirishima has to accept and embrace the fact he has feelings for OC. Meanwhile, OC has to accept and embrace their feelings for Kirishima. Once these requirements are met, the red string will snap into place.