That's What We Call Love「それをボクらは愛と呼ぶ Sore wo Boku-ra wa Ai to Yobu」 is the 213th chapter of the manga, Nanatsu no Taizai.
As the fight between the humans and the warrior of Stigma continues, Gerheade tries to have the two side to stop fighting. Rou at first tells her to flee or be killed then, after she steps in to protect a fellow fairy from the human's wrath, tries to prompt her to kill him. Before she can even speak, another human kills the male fairy and fatally injures Gerheade. After Rou kills his fellow companion, him and Gerheade speaks of the reason of the humans' betrayal.
Years before, Rou's home village rescued a demon collapsed on the road. After a few days they wer taking care of him, Stigma attacked the village and killed all of his inhabitants, including a childhood friend of Rou who looked a lot like Gerheade. Only Rou and three other boy survived, and from that day they trained to exact their revenge on Stigma.
Gerheade then promises him to explain the situation to her brother to protect him from the Fairy King's wrath, and then speaks of an unknown feeling that warms her heart, sprouted from an illusion of Rou taking care of her.
Shortly after King, in the body of Gloxinia, enters the Fairy King's Forest. Gerheade at first tries to stop him, then to have Rou flee. The human is, however, firm in taking the Fairy King's attack, and after telling Gerheade to survive is targeted by Basquias.
The fight between the humans, led by Rou, and the warrior of Stigma continues. Gerheade tries to have the two sides to stop the sensless killing, but the only outcome of her pleas is to be called a coward by a male goddess. Short after that, the goddess is killed by Rou.
She then addresses directly Rou, who tells her to flee or be killed. He then is hurt by a fairy, but before he can exact his vengance on him Gerheade steps in to stop him. Rou tries to prompt her to kill him, but before she can even speak a word another human kills the other fairy, slashing her face at the same time. Gerehade tries to crawl away but the human keeps hitting her.
When Gerheade regains consciousness the human is dead, impaled by Rou's sword, and Rou himself holds her in his arms. When asked why she did not kill him, Gerheade tells him that she felt, from within Rou, sadness and anger, and the image of a girl with a striking resemblance to Gerheade herself.
Rou explain that she was a girl of his home village, and childhood friend. One day some of the villagers ran into a demon, who Rou describes as a good person, collapsed on the road, and rescued him. A few day after the rescue some warriors of Stigma attacked the village, killing all the villagers but Rou and three other boys, who went training on the mountain. The four then honed their skills, until the day they could have their revenge.
Gerheade tells Rou that she also committed an unforgivable sin, by averting her eyes from Ludoshel's plan, and promises to explain the situation to her brother to avoid the fairy king killing Rou. She then speaks of a feeling unknown to her, that has sprouted in her heart after seeing an illusion of Rou taking care of her instead of his dead friend.
The speech is interrupted by the appearence of King. Gerheade first tries to explain to King what happened, then asks Rou to run. He replies that he is fine with what's about to happen and, after telling Gerheade to survive, is targeted by a full power Basquias blow.
Q219. Tanpo-san: How come King and Elaine don't have pointy ears like the rest of the Fairy Clan?
Nakaba: Not all Fairies have pointy ears.