Durarara!! the anime is...
...despite the trappings (headless horselady, gang war, slasher mystery, slice of life in one of the busiest metropolises in the world) really just the story of one incident in the extended courtship of Party S, by Party I. Honestly, can't he just send flowers? You can't go wrong with flowers. Unfortunately, I think something as straightforward as flowers might drive Party S more crazy trying to figure them out than all the trappings above. Put together. Which would totally be a plus for you, Party I!!!
There is undoubtedly a doujinshi about this somewhere. Haha.
Edit: ...scary thought. If Izaya has sisters, must he also have parents... somewhere?
There is undoubtedly a doujinshi about this somewhere. Haha.
Edit: ...scary thought. If Izaya has sisters, must he also have parents... somewhere?