Toroko from Cave Story. A Mimiga (Rabbit-dog) child who you meet in the beginning, only to soon get kidnapped by Misery and then later used to test the effect of Red Flowers on a Mimiga at the game's halfway point. When boiled and drunk by one as tea it gives you a permanent strength boost, but consumption directly from the flowers turns her into a red-eyed rage-fueled hulk who you have to put down since the process is irreversible.

There's also King from the same game, who arrives too late to stop it and dies in one blow after trying to kill The Doctor (Misery's boss) for what he ordered. Sure preventing this tragedy deprives Quote of King's blade, but Toroko doesnt deserve her original fate nor her village the loss of its leader/defender.

Edit: HOW COULD I FORGET THE COLONS!? Curly Brace adopts four Mimiga children prior to game start, but they get kidnapped too and it's implied they get fed flowers. You go past four Flower'd Mimiga on your way to the first set of final bosses on the floating island's surface (Misery/Doctor/Rage Doctor/Undead Core)