Is there any place to find backstory for this game?

The instruction booklet of the Sega Saturn version has short bios for each of the playable characters, but other than that, I don't know of much else :/

If you want me to, warn me and I'll type them out for you.
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Now you'll REALLY be able to see why I am a Boss character, and not some thug!" - Kanon, after learning of Guardian Heroes for XBLA

From the booklet:

Han
Originally a member of the Royal Knights Blue Team, he decided to head out into the world and become a wandering adventurer. He carries one of the most powerful swords ever forged, a legend in itself.

Randy
Randy has been studying sorcery for many years. While taking a break from his studies, he met Han and the other Guardian Heroes. He decided that the best way to hone his skills was by travelling with the other heroes. His companion and fellow sorcerer is Edward M. Nando (you may call him Ed). Ramdy is a shy fellow, but he hoprews to overcome this by adventuring with the Guardian Heroes.

Gingirou
Gingirou is a ninja travelling in search of the legendary sword Muramasa, which is supposed to be so powerful that it has acquired a soul. Because of his desire to see this legendary sword, Gingirou decided to join the Guardian Heroes team.

Nicole
Nicole is a mischievous but devout cleric. She has gone with the rest of the Guardian Heroes believing that it would be an interesting change of pace. She herself is an optimist and tries to see only the good in people, but it's best not to make her too angry, as she is also capable of casting some very powerful spells. Perhaps she's just tricking everyone into thinking she's a nice girl. Perhaps not - she's full of secrets...

Serena
Commander of the Royal Knights Red Team, Serena has decided to side with the Guardian Heroes. She seems to know the sword Han has found, but has yet to reveal her information to the heroes. Who is she really, and why is she helping the Guardian
Heroes?

Undead Hero
This mysterious knight was brought back from the grave by a power far greater than life itself. Now on the side of the Guardian Heroes', the Undead Warrior follows their commands.
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Now you'll REALLY be able to see why I am a Boss character, and not some thug!" - Kanon, after learning of Guardian Heroes for XBLA