Phantom Navis are essentially a duplicate of the Navi in question, taking on their properties. They include physical appearance, voice, memories, and even chips and moveset. However, the accuracy of the copy appears to depend on the amount of data stored in cache. For example, the copy of Bass had amnesia, and although powerful, MegaMan noted that he wasn't as powerful as the original.
In the game, the personality of cache copies appear to progress as time passes. The first copy seen by MegaMan only roared, and early copies encountered were agressive, often saying the word "delete". They only talked in katakana. Later copies began being able of dialogue, with ShadowMan speaking in a rough way and Bass talking almost normally.
Those copies began causing trouble around the world, with Lan and MegaMan thinking that the WWW might be responsible when they fought FireMan in Mayl's home. Later, they are surprised to fight against IceMan after saving the real IceMan in the WaterWorksComp, with Dr. Froid noting that IceMan had not connected to the internet after his rescue. After finding a copy of WoodMan, MegaMan and ProtoMan wonder if the incidents are connected and ProtoMan takes a piece of WoodMan's data to be analyzed on SciLab.
Early research from Dr. Hikari suggest that it is some kind of backup data, but he thinks it may not be the case as it would be impossible for a backup to be taken without the operator noticing, and he notes that the data was manipulated for malicious purposes. After ending the analysis, Yuichiro discovers that it is cache data, explaining why copies of Navis appeared without the knowledge of their operators, and SciLab investigates cache servers for clues. However, there have been no records of unauthorized access to cache servers. It is later discovered that Cache was responsible for the copies, making them from an old, abandoned cache server that was used in the past by Dr. Wily. Cache is defeated by MegaMan, putting an end to the cache copies.
In the anime, Phantom Navis are identical to their original counterparts, being only distinguished by the multitude of rings that appear in their eyes and their evil personalities. Phantom Navis are able to materialize in the real world by sheer will, without explanation. All Phantom Navis share one weakness: They will disappear if they are damaged, even slightly. Their existence was discovered by SearchMan when he was attacked by copies of MegaMan and ProtoMan.
A shadowed out Phantom Glide attempted to ambush SearchMan.EXE, but SearchMan noticed him and shot him down.
Phantom GutsMan attacks SearchMan along with Phantom IceMan, Roll, Glide, Rockman, and Blues.
Phantom Roll attacked SearchMan with the others, but was shot down by him.