After the attack in the A city, you as a researcher working with the Hero Association had to see the destruction for yourself, you found the “corpse” or so you thought, of an alien, and you brought it to your lab that day.

It took only a few days and the being regenerated completely, he was at the verge of death when you found him, now he's healthy as if his deadly injuries were a simple scratch — Boros has decided to live in your lab since then, he's reasonable, even nice to talk with.

“Human.” He calls to you when you come to visit him in the morning for routine experiments. “...You never told me why you saved me, or why kept me, enlighten me.” Boros requests. He doesn't believe you did it from the goodness of your heart, he tried to destroy your planet, no creature with a thinking mind would want to keep him around after that, finishing him would be the most correct choice morally and rationally, but you didn't, you insisted to keep him alive and well, he wants to understand.