“Signature” or “Signature Magic” refers to your entirely unique magical ability. All Guardians have one, but no two are the same due to no two individuals being the same. While your Signature Magic is still technically magic, it is not something that will ever be truly replicated in a Grimoire. Signatures are incredibly powerful and capable of almost anything from actually stopping time to temporary invincibility. They do not function by the normal rules or limits of magic.” Selene ch.5
Darks get ones that are stronger then normal and are synergized with their other Origin bonuses
Yellows get ones that are very unique and capable of breaking what should be possible with magic.
All signature magics have a true name, which very few people know, and a code name, which is common knowledge.
An extensive explanation of signature magics is given in ch. 36.
Notable examples:
True name: Kodoku, aka こどく.
Code name: not yet picked (as of ch. 37) aka こどく
The hiragana is significant, allowing the word ‘kodoku’ to be a homonym with two different meanings:
“Kodoku […] can mean loneliness, solitude, or isolation. […] Kodoku is also a name for a type of poisonous magic found in Japanese folklore.” (Bookbear, ch. 37)
https://en.wikipedia.org/wiki/Kodoku
There is a long exposition by Bookbear about Mai's signature in ch.37. Summarizing, it gives Mai a miasma resource (negative mana) at the cost of her health. This gives her curse magic, which is effective at destroying normal magic, Applied to bullets. it increases penetration, does damage over time if the bullet stays in the target, and makes regeneration of any wounds harder. For more details, see ch. 37.
True name: Razorglade.
Code name: Nature's Blades.
“My Signature Magic allows me to transform any plant life into a unique, powerful metal that I can then freely manipulate at the cost of my mana.” Everglaive, ch. 36.
(At one point Glade of Glaives, Ever's finale class spell, was incorrectly described as her signature magic. It isn't.)
True name: Rills of Restoration.
Code name: Rejuvenating Waters.
"It lets me enchant water in my surroundings, giving it regenerative effects and letting me control it. It also works on spells based on water too, which makes my water-based healing magics much stronger. Naiad, ch.36.
His signature Magic is a charge type. By continuously dancing he can build up energy to use for various things such as a melee attack that releases stored energy. If he stops dancing for any reason he loses the charge. There is a finite limit on how much energy he can store.
Code name: Second Chance.
“Using it, she can revive herself from death at a predetermined location so long as she has enough mana. ” Everglaive, ch. 36.