Unlike BoRK, Kindred's passive on-hit can't lifesteal, isn't capped vs monsters, minimizes at 640 HP
I think this, Aatrox's W, and J4's passive are the only physical damage on-hits that don't proc lifesteal, and I'm fairly certain it is the only %HP in the game not capped vs monsters.
Her on-hit deals 1.25% current HP in physical damage, with a minimum of 8 damage per stack - that means against anyone with less than 640 HP, it deals damage as if they had 640 HP. The minimum damage IS NOT in the tooltip, but is very evident in game.
denting blow damage doesn't proc lifesteal unfortunately.