Sona
死神
I just did, i only skimmed the history, but the rest i did read and i now ship Isabella and Sona.
Ok, but seriously, i must assume they would get along great. Not only have they very similar personalities, being very rational, calm, unreadable, cold. They also both enjoy being in a quiet, fairly isolated environment. What im getting at is, that i can imagine them just sitting together, each doing their own stuff, not talking and having a blast. Like, they understand each other on a very deep level that the others cannot.
Maybe you see that differently, though, let me know.
I updated the background history between Isabella and Sona, tell me if you like it or even if you have a better idea.