Entangled with You: The Garden of 100 Grasses (manga)

Once upon a time, Marcel's sister fell in the forest while gathering herbs. Her injuries were grave and Marcel was at a loss for what to do, until a fearsome horned man appeared, offering help-in exchange for a promise. Now, ten years later, Marcel returns to the woods, steeling himself for death.

Instead, he finds something quite different: a gentle soul, friendship...and perhaps something more.