I thought that Masamune was both the sword and the blacksmith that forged the sword. And that japanese blacksmiths often named their weaponry after themselves. There's the story of the Masamune and the Murasame and how the former repels leaves flowing down a stream while the latter attracts them and meanly slices them up.

And with Shadow, he'll just do that disappearing thing on you for a while in the World of Balance. When you hit World of Ruin, he won't leave you again. Just remember to wait for him when you're at the Floating Continent instead of ditching him and jumping back to the airship pre-emptively.