Are Watari and Kaori a couple by the time she is hospitalized after missing the gala violin concert?
At least judging from the Anime Series, they never have been a couple.
Here is why I believe that:


Just after the group of friends went to visit Kaoris concert for the first time, Watari does notice that Kousei has a thing for Kaori. I mean the dude is a piece of bread but even he noticed.
He does hardcore wingman Kousei to go see Kaori once she's in the hospital.
Apart from the fact they have nothing in common he still seems too interested in all other girls as well, thats how I'd judge him not really bonding with Kaori in the first place. :)