The Glass Coffin {Il Dottore X !Little Sister!Oc}

Having contracted a deadly disease, Dottore sedates Mizuki into a coma-like state and puts her in a glass coffin to keep her in a controlled environment until he can find a cure. After many failed attempts to save her life and years wasted not being able to make memories, Dottore's heart now has a wall built around it that only one person could break down...

His little sister.

How is that the walls did come crumbling down? Even when she can't be awakened easily.

Note: Dattore is originally from Sumeru. But in this, his family moved to Snezhnaya when he was young sometime when he was about fifteen and Mizuki was thirteen. Mizuki got the Winter Frost Illness when she was sixteen and Dottore used the knowledge to make something to slow it down before finding that he could put her in a coma to give him time to find a cure. He leaves Mizuki in the care of someone he trusts before heading off to Sumeru in search of a cure as he goes to the Akademiya as a cover to do so.

I rewrote this as more lore of Dottore came out making him over 400 years old.