Well, Odashima (the blonde girl) did nothing wrong. Neither Ohta or Kashiwada is in relationship, nor they said like to each other. It's all fair and square. She just... moves faster.
*picks up phone*
'Hello Kashiwabro, it's me. Yes, I'd like to borrow your... services. It's for a blonde girl wearing purple. Yes, I'll give you the coordinates of the target in one hour at the usual place, like last time. Ok, thanks.'
"I don't have any ulterior motive or anything"
Well yes, but actually no
well if she really likes Ohta I hope she confesses, doesn't matter if she gets rejected. That would be the most amazing moment if it happens plus character growth!