ENDING... STUFF.
Endgame:
So Ryoji finally awakens to his full death powers near the end of the year. December 31st he offers SEES the choice. When they refuse to kill him, he runs away and hides out for pretty much all of January.
January 31st, there is the final boss battle with the Avatar. Things look bad, especially since Minato and Minako are holding back a little because they don't want to hurt Ryoji.
... And then Ryoji shows up, minus his scarf. He explains that Death left him the other night, and he tracked it. When they ask how he got through the Shadows, he just smiles and steps into the battle lineup.
And summons Thanatos, without an evoker.
The battle continues as normal, and the Avatar goes down.
Minato and Minako fight Nyx, but before they can do the Great Seal, Ryoji appears again.
"Let me become the Seal."
"What?!" "Why?!"
"I should have died a long time ago, on that night. You two have a chance to help the others more."
They all hug, and cast the Great Seal, with Ryoji as the soul holding it in place.