like how did she get punched?
the asshole raises his fist to attack, ritsu pulls her away from him
then she freaks out, ??? happens, ritsu isnt protecting her anymore
then she has a bloody nose and the asshole is making excuses
When it comes to the friendzone, think about how a native English speaker would say it in English.
So as long as they're keeping it casual, just go with "Sis" or "Sister" instead of spelling the whole thing out every time. We already know what her name is.