The first time Haruichi tastes alcohol is from his brother's mouth.
The liqueur was a good idea - Haruichi can smell how sweet it is, like bubblegum and cherries in equal parts - but there's a burn even just from the careful sniff he takes, a evaporative heat that closes his throat up just from the idea of letting it touch his tongue. But even if he can say no to the full shotglass, he knows he's doomed when he sees the way Ryosuke smiles, even before his brother tips his head back and swallows his own drink in one smooth mouthful before setting the glass down and reaching for Haruichi's. He keeps his head up, this time, opens his eyes just enough for Haruichi to see the color shift into darkness as the pink liquid slides past his lips to pool on his tongue.
Haruichi is shutting his eyes even before Ryosuke's fingers curl against the back of his head, parting his lips in willing submission before the burn of alcohol clinging to Ryosuke's mouth touches his. It's as bad as he expected, syrup sweet and alcoholic burn in equal parts as he swallows the heat down into his stomach, but Ryosuke lets his touch linger, keeps his lips parted so Haruichi can lick the lingering flavor off his tongue directly.
Haruichi doesn't know how Ryosuke can stand to hold the alcohol heavy on his tongue for the time it takes him to lean across the table each time. But every time Haruichi swallows back the alcohol a little more of Ryosuke lingers on his tongue, like the burn in Haruichi's throat is branding his brother into his body more than the alcohol. He keeps going, accepting the poisoned almost-kisses until he can't stand up, until he's kneeling on the floor and clinging to Ryosuke's shoulder just to stay upright.
He remembers Ryosuke's laugh, in the haze of bleary intoxication, the pressure of a hand at his back urging him into the bedroom so he can collapse fully-clothed atop his sheets. He almost remembers the fingers that stroke through his hair as he slides into unconsciousness, the touch uncommonly gentle against his skin. It's lost to a dreamscape by the time he wakes with bitter on his tongue and an ache in his head, but he wakes up smiling even as the details fade into forgetfulness.