Ken and Daisuke accidentally wind up messing around while Ken is wearing a specific clothing item (choker/necklace? fancy scarf thing? etc), and then he starts to wear it as a signal to Daisuke when they can just sneak away and mess around. They never talk about it, never mention anything otherwise, but Ken keeps wearing it at increasingly inappropriate times because he's a fucking babyslut who's addicted to Daisuke fucking him and enjoys teasing Daisuke. And Daisuke could never resist him if he tried. Eventual confession, obviously.