I can rec you a good TatsumixWatari fic. Except... its also MurakixWatari (in a pretty much entirely consensual way strangely enough). Watari is wonderful in it... very smart and dark. And Tatsumi is lovely. There are some sweet moments between them ^^

It's called The Doll, but there's also the Grey Knight series which is rather sweet too ^^

http://katcom.squidkitty.org/Fanfiction/Laekin.html