"NX" is a series of cards found in, and is the namesake for Future Card Buddyfight NX. Many characters in NX utilize cards from this series, and "NX" cards are found across many worlds.
The mysterious power of "NX" represents the untold hidden potential within a monster. Ichirou Toyotomi is one of few people who know of this power, and attempts to unlock it and utilize it for his own ulterior motive.
"NX" is actually a codename used for the ability. "NX" is short for Nexus Access (bit of a stretch, if you ask me), possibly referring to the connection (Nexus) between Buddies needed to unlock (Access) the hidden powers seen with the "NX" phenomenon. It is said that only the truest of true Buddies can unlock this ability. As such, only a very few monsters have been identified as possibly containing "NX".
List of "NX" Cards
Darkness Dragon World
Star Dragon World
- Heavenly Helix Dragon, Ellie "NX"
- Supernova Dragoner, Jackknife "NX"
- Undying Abyss Helix Empress Dragon, Devina "NX"
(More to come later at people's suggestions)