Gon X Killua Love

When you give a fire demon an unlit candle, it shows trust. If you give them a burning candle, it's a sight of love, as if to say "You have my heart". It can show you deeply treasure a friendship but it's far more used between families and lovers.

Killua is the only kid to be a fire demon like his dad, most of his siblings inherited their mother's power over shadows. Illumi and Silva were in charge of his training, so Killua is wary of darkness and hyperaware of how dangerous fire can be.

Killua's necklace is an enchanted Zoldyck relic that keeps him attached to the living world, if he breaks it or loses it he will be forced back to the underworld, where demons belong. He is weaker and loses some of his demoniac features outside cursed places (the underworld, summoning circles, and such), but he is still powerful.

The amulet needs a blood offering to work, it can be a demon's, human's, or animal's blood, if it doesn't receive it, it will take a good chunk of the user's blood.

Killua thinks the living world is beautiful, especially the sun and the sky, but knows he can turn things into ashes easily, so he holds back on his fire powers as best he can. When he makes an effort to stay calm his eyes are dark grey, they become blue when he is comfortable or uses his powers, and can become white in extreme situations.

Gon summons Killua on accident and asks him to be his friend, which Killua accepts, not reminding Gon a pact with a demon means he can take his soul in exchange later if he so desires.