I've already beaten it and gotten the "pacifist" route, so you can fight a select few girls until you are given the option to mercy, however some must be fought until "death" which is actually the same ending dialogue as if spared along with girl fleeing.
1st fight (maid) can be spared through fighting, 2nd fight (farmer )can be talked to about flowers, can not be mercy'd through fighting, 3rd fight (rogue) can be spared, must be fought normally to mercy, 4th fight (chief) must be defeated normally no mercy option, 5th fight (archer) can be spared must be fought, 6th fight (blacksmith) can be spared must be fought, 7th fight (pirate) can not be spared must be fought normally.
TLDR; 1, 2, 3, 5, and 6 can be spared, and doing so on the island after fight 7, you can find a dagger behind vines on your way up, which will unlock the true ending after beating the 8th boss