Poison
Kahoko Hino is a poison. Len is sure of it.
Poisons kill you, slowly or quickly dragging you into the darkness they provide. Poisons can mess with your mind, inhibiting your neural synapses so they don't fire and your brain doesn't work. Poisons can disrupt your heartbeat, making it fluctuate wildly until it stops. Some poisons sear through your veins, painfully and slowly killing off your cells, and some poisons release endorphins, making you feel almost happy as you're eaten alive.
Len isn't sure if it's possible for one poison can work in all such ways. But if it is, Kahoko Hino is definitely one of those.
She's killing him. Having her around makes his mind stop, toxins coming in through his eyes. Being close to her makes his heart beat wildly, the scent of her hair making it quiver as she dances past. Seeing her cry brings him to a full stop, agony at her distress wracking through him, and her smile makes him flush as she infect him even more.
Kahoko Hino has poisoned him. Len knows he will die if things are left as they are – wasting away, infected, infatuated, unable to acquire a cure.
But while she is the poison, she is also the antidote.
And Len doesn't really want to die.