Guardians were creatures that lived in RaDos before the death of the Primordials, but now exist within green Di-Gata stones called Guardian Stones.
They are made of pure energy, and they vary in size, appearance, and abilities. Some casters consider Guardians as friends, and others as servants. Most are bound to a Sigil, but some are bound to henges. Most of these sigils or henges are engraved on a part of the guardian's body.
Some casters say a short phrase when summoning their guardians (most notably the Defenders). Whether this is required or not remains to be seen. Commonly, when a Guardian is defeated or has been out for too long, it returns to its stone. Guardians are only killed when their stones do not return to the caster, or if it shatters.
It's been shown a few times that guardians can be merged together to form larger, stronger guardians. The method through which this is done, however, is largely inconsistent. The first time we see guardians merge was when Kragus and Omniaxor merge to form Omnikragg. This was done willingly by the two guardians, to save Kragus. This shows guardians can merge together of their own free will, without human intervention.
It's also show that if guardians are torn apart, the pieces can be used to create new ones. When Brackus was banished to the Dark Realm, Anaconduit had joined him. However, it later got taken by Si'i and rebuilt as Darkviper, a by-product of a merge between Anaconduit and another guardian. Even though they are made from pieces of destroyed guardians, they maintain their memories, as Darkviper recognized Brackus. Darkviper should not be confused with a mutant guardian, he was formed through two living guardians merging rather than a jumble of dead parts.
It is possible for a caster and guardian to merge. We see it first when Kragus protects Seth from Omniaxor. Omniaxor accidentally merges them together, and this causes Seth to grow rock like armour over his body, and have greatly increased strength. However this also made him somewhat emotionally unstable, but he soon learns to control it. They are later seperated by the Megalith.
Another example is later found in King Magnun's merging with his Guardian Voltanis to extend his mortal live turning them in two a gaint, two head mummy-like monster.
Much later in the series, after an accident in their Dojo's reactor, many of the guardians gain the ability to merge and un-merge with their specific defender at will, providing many useful traits such as increased strength, and even the ability to fly.
Mutant Guardians Edit
Mutant guardians are artificial guardians created by Si'i using the Guardianizer. However, the guardians it created were psychotic, and couldn't distinguish friend from foe. It was used on the Defender's guardians when Si'i was found out about his illegal operation (It was illegal in RaDos to distribute or sell Guardians unless you had some affiliation with the Gata-shin Monks or Wizards of Yan).
The Guardianizer was later modified to have its effects reversed, saving the Defenders' guardians from a permanent mutation.