AFAIK there are no spoilers - this chapter came out 3 days ago, and the manga is an original.Guys I need some spoilers.
It's most probably a hickey, or at least that's how Riku saw it. There's the possibility that it's a legit wound that's unrelated to her relationship, but given his paranoia and the context of their parting Riku can't help but interpret it as a result of them making out.Did he do something or order Shizuku to wear band-aid on her neck?!