The ending animation has Marin and Urin seeing the story of the Little Mermaid, with Urin crying over it and Marin comforting her.
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 and Kanon continue to do this for every other monster in the show.
Marin and Urin playing with bubbles in the shower.
The flashbacks of a young Marin and Urin having fun, and meeting each other after Urin was first born.
In episode 9, Kanon realizes that it was her own insecurity that made her feel like Kojima was going to dump her, when in fact he had liked her all along. At the same time, Kojima confesses to Marin that he had liked Kanon from the start, but felt she was purposely separating herself from him, and so they grew distant. However, he feels that when things work out, he'll be able to see Kanon's smile again. Kanon realizes that she never told anyone she liked them out loud even once, and heads off to find Kojima and tell him she likes him back.
In the final episode, Urin begs Marin to hate her so she won't die when Sedna releases her power. Marin responds that nothing could ever make her hate her sister, and doing so would be the most painful thing in the world. It's set to this music.
The last episode. Instead of defeating Sedna with a spear of light and killing Urin, Marin and Kanon reach out to her. They hug her, and the power of Marin's love and Kanon's 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.