Masamune explanation: I just spent 2 hours researching all this so I hope questions are answered!!

the explanation in the game is very, very wierd. as you know masa and mune inhabit the masamune. their big sister is doreen who looks after them. masa and mune (aka Grand & Leon--thus the GrandLion) are effectively neutral alignment but doreen is good. doreen allows them to inhabit the sword so they can help on the quest (They inhabit the ruby knife ((Chrono Trigger)) in order to trash the mammon machine, but it gets broken and is later reforged into the masamune for frog to use against magus) Masa and Mune somehow go evil before CC (In Chrono Trigger Masa and Mune mention that it's not about who owns the sword, but about how it's used ((In CT you learn in the kingdom of zeal that the Masamune takes the form of the heart of the carrer, in Cyrus case he had a broad sword so it took that form, and with Frog since he was trained by Cyrus and fought with a Broad sword it took that form for him, and with Serge it took the form of a swallow)) Thinking upon this I thought that after the events of Chrono Trigger, when Frog was sent back to his own era, that he decided that there was no need to have such a weapon and decided to put it back where it belonged. During the course of the 400 years that separate Serge's era from Frog's someone must have used the sword! for an evil purpose, and therefore the Masamune became a dark sword. They get too caught up in the sword's personality, and a sword is for killing things, ). later on when you defeat Dario, Doreen finds Masa and Mune and tells them off for being evil, and inhabits the sword as well, as a moral guide for masa and mune. then the masamune becomes the mastermune and you can equip it.

MASAMUNE, The Greatest Sword Smith Of All Time
Masamune's familiar name is Goro Nyudo. He lived around the 13th century (Kamakura period) and died sometime in the 14th century. Masamune is the best known sword smith in Japan of all time, and a well-known philosopher

MURAMASA, The Cursed Sword smith
I actually had a hard time finding information on Muramasa, so there's not a whole lot I know about him. The other sword smith named Muramasa was a Japanese student studying the art of sword-making. Muramasa lived during the 16th century (Muromachi period). He wished to recreate Masamune's work, but he failed. In the Japanese bujutsu there is a saying: satsujinken katsujinken. Roughly translated, this means 'The sword that kills, the sword that gives life'.

MURASAME AND THE MASAMUNE, The Cursed Combination
An anime called The Hakkenden, The Legend of The Dog Warriors uses the Murasame in their story. The Final Fantasy series, Chrono Cross, and a few other games have this sword somewhere in them. So what's the big deal with these swords?

In Japan, two "magical" blades exist. These are Masamune (Mah-sah-moo-nay) and Murasame (Mer-ah-sah-may) swords. If you put the two blades in a river with floating leaves, the Masamune blade would repel the leaves and let them flow safely down the stream, while the Murasame blade would attract and cut them up. The Murasame blade is supposedly cursed, but very powerful. It is so innately cold, that water disperses off it when drawn and exposed to air. This test symbolizes the two periods and how chaotic the Muromachi period was.

Hope that helps