Hayley's Earing
Zoro’s blood oath to protect Hope until she could defend herself as an unborn child in Hayley’s womb caused a blood-red crescent moon mark to appear over his heart as a physical reminder of the oath he made. This oath gives him the ability to sense where Hope is at all times, like an extra tail or arm, even a world apart. When she’s still unborn, this sense extends to Hayley since she carries her.
Zoro’s song for Hayley and Hope that he’ll play on the Niko for them is A Thousand Years. It’s his way of telling them that he loves them since he’s not the type to say it out loud very often. When Hope was a baby he’d play it for her to put her to sleep and during the time that Hope was taken away from Hayley played it for her to remind her that one day Hope would be back.
The two of them are constantly marking each other with bite marks and their scent. Only in Zoro's case, Hayley's bite marks stay longer on his body, so sometimes he likes to call himself her chew toy just to rile her up. Which usually leads to more marks appearing.