But on the topic of spirits...
This segways nicely into what I was about to make a topic of my own about.
Most people take it for granted that Masa, Mune and Doreen are the spirits that are found within the Masamune. Masa and Mune inhabit the sword, while Doreen shows up in time for Cross... somehow.
I've personally never been satisfied with the simple explanation that they're nothing more than a personification of that weapon. For one, we know Masa and Mune existed before the actual sword did, because, chronologically, you meet them before you get the Ruby Knife. And the actual Masamune doesn't even come into being until the power of Lavos merges with the dreamstone knife. And Doreen doesn't seem to be attached to any physical object whatsoever.
And that leads us to the nature of the Masamune. Most people believe that it is a sword meant to strike down evil. That assumes a more or less objective definition of what
evil is. And since I don't like to consider Chrono Trigger a fairy tale (its sequel certainly isn't), I think there's more to the Masamune than that. So why is it so effective against Magus?
Myself, I think Masa, Mune and Doreen all used to be human. I believe Masa and Mune, in their youth, were friends with Janus. But a bitter rivalry tore them apart (not sure exactly what that would be. Maybe Janus got jealous that Masa and Mune were so proficient in magic), which explains why the Masamune would harm him but not any other supposedly "evil" enemy (such as Slash, Flea, Azala, Zeal, Yakra XIII or LAVOS).