"Show a human a little mercy and next thing you know, he thinks himself your equal! … I've let a mere boy make a fool of me for the last time. You stand before a demon, or should I say, a weapon without mercy!" – Ghirahim, The Legend of Zelda: Skyward Sword
It was easy enough to fool a human, especially the Goddess's chosen hero. He was impulsive, reckless, a warrior. Warriors depend on their warrior instinct; the split-second assumptions made in the heat of the battle guide a warrior's blade. As the living embodiment of a weapon, Ghirahim knew all too well how to dazzle a warrior.
However, Ghirahim had no control over the collateral assumptions the hero would make. Every war had its unfortunate civilian casualties, but Ghirahim never imagined that such a loss could befall the coveted weapon of Demise. Link seemed to be under the impression that the Demon Lord was a man. Sure, Ghirahim had fashioned an entirely new layer of skin to fit in with the surface dwellers, but clearly the sword at least appeared feminine if not androgynous! Purple eyeshadow, glimmering diamonds, and a skintight jumpsuit were certainly not hallmarks of masculinity.
It couldn't be because of Ghirahim's flat chest. The Goddess's wretched servant dog Impa was as flat as a board and just as stiff, but Link recognized her as a woman. Maybe Link just hated women, so he referred to Ghriahim as a man out of deference? No, if the hero truly respected the Demon Lord, he would at least refer to Lord Ghirahim by the proper full title before swinging his sword around madly.
There could be only one reason Link insisted on holding Ghirahim to the human concept of gender. It was because of mercy. Ghirahim had spared Link's life twice, hoping he would give up and retreat to the sky. That mercy led to the Goddess's chosen hero thinking he was equals with the Demon Lord.
A sword had no inherent lust for blood. A weapon was merely a tool of its master. But with Demise imprisoned, Ghirahim had no choice but to embody a weapon without mercy.