To "go ghost" refers to the ability to interchange between a mortal and ghost form at will. It is only possessed by half-human, half-ghost beings. It is considered a rare type of power to have.
Within the original Danny Phantom series, Danny is the first person to use this ability in "Mystery Meat," in which he changes willingly into Danny Phantom during his first battle with a ghost. However, he has limited control over it and accidentally returns to his human form several times before finally learning to maintain his ghost form. Over the course of the series, he is shown changing back to Fenton when drained of energy, knocked unconscious, or forcibly transformed by more powerful beings. He also demonstrates control over how the transformative glowing rings appear during mid-morph (split horizontally from his torso, split vertically, from hand-to-hand horizontally, etc.).
The second character to exhibit this rare power-type is Vlad who, in his debut episode, reveals that he is the "Wisconsin ghost" that defeats Danny Phantom when the opportunity is most to his advantage. Pending more...
Within the Danny Phantom: Spectral Connections series, there are three Halfas with this established ability following from mainstream media.
- The first introduced in the series is Danny Fenton. Having gained virtually total control of all his basic powers, and with everyone knowing his secret identity as Danny Phantom, "going ghost" is more of a second-natured action rather than a fugitive ordeal. However, he still cannot control the fact that he returns to his human form whenever knocked unconscious, but he hopes that one day he will.
- The second introduced is Dani Phantom, one of Danny's clones, who was created with approximately the same level of control as Danny at the time of her creation (almost a year into the canon show's timeline). She reflects the nonchalant attitude he has towards "going ghost" in public since he revealed his secret identity, which allowed her to do the same. And with his open support, she has been welcomed by the world and celebrated for being one of the ghosts that helped him save the world. Pending more...
Sam Manson: Monster Hunter
Within the Sam Manson: Monster Hunter series, Danny can interchange between his Fenton and Phantom forms.