In the second episode, Kanon and Marin are attacked by their first Monster of the Week, a woman with tentacles for hair. When they combine their attacks and go to finish her off, Marin suddenly stops and says she's seen the monster's eyes before. Then she runs up and hugs her, and it's revealed that the monster was a transformed octopus, and that her hug has changed it back to normal. Rather than kill the Monster of the Week, she saved it.
Marin continues to do this for every other monster in the show.
Marin and Urin playing with bubbles in the shower.
The last episode. Instead of defeating Sedna with a spear of light and killing Urin, Marin reaches out to her. She and Kanon hug her, and the power of their love and acceptance of darkness moves Sedna's heart and causes her to disappear peacefully.
The aesop of the series. Everyone has darkness in their hearts. That's okay, and we must learn to accept it, not cast it out.