Zelflux is a Titan created by White Goddess in case of severe emergencies. He hasn't been awakened for millenniums.
He is created out pure energy, causing anything to touch him to vaporize. However, unlike his brother Reptflux, the core has fused with his main body. If the polarity is reversed, he will no longer be able to move, and will be turned into pure Cobalt.
While in Cobalt form, he is in a stand still, however other Titans can use him as a strong weapon.
Fighters of Lapis
Zelflux appears in the game, although he is much smaller and less powerful. He doesn't vaporize people on touch, although he has powerful energy blasts from his core. He has decent recovery and decent power.
In the post-credits stinger, it is revealed to be a Tabuu spy. It is unknown how Tabuu managed to duplicate Zelflux and Reptflux, or even if it was him that duplicated either one.