Synthetic nymphs are created by the Black Crusaders for the sole purpose of giving nymphic powers to humans. Their creation methods are the Black Crusaders' well-kept secret - They inject nymphic blood into various animals to create unique powers. They discovered that every living thing in their plane of existence has at least a small amount of magical ability, and injecting them with highly potent nymphic blood brings out that ability, but kills them in the process. However, after the animal has synthesized with the nymphic blood, it's own blood can transfer powerful abilities to humans through another injection, and significantly fostered by Anthracyllin and/ or Anthracaine.
Unbeknownst to the Crusaders, not only did they create injectable abilities, they created bodiless, conscious nymphs that desperately try to take control of the bodies they inhabit. These nymphs appear as the animal they were derived from within the host's Dreamscape.
When the nymphs gained control of their host, they inherit the host's memories and behaviors, however, they would slowly notice that they were not themselves. Some even rename themselves, not exactly understanding that they had become the nymph that inhabits them. It behaves like a slow infection to the host, and slowly takes them over. Most of them end up manic and unstable if they are not killed first.
This is not always the case though, as each synthetic nymph is distinct. For example, Soren's nymph, a butterfly, is extremely weak, and its abilities are completely non-physical. It manifests itself within Soren as an attachment to butterflies, rather than completely overthrowing him, like the fox nymph within Lee.
Vi is the only one to receive a true synthetic nymph, as her nymph was derived from her brother, Io. She was the first, and most powerful, successful synthesis. However, she does not have any flashy abilities like the ones that follow her. She was given eternal youth and rapid healing by the transfusion, which is handy but, in X's eyes, not useful enough. The specific and risky methods required to create one like Vi's are what inspired the use of common animals in nymph synthetization, which proved much more successful.
Another anomalous reaction to the injected nymph would not only be overpowering it, but noticing it as an intruder and killing it. This can only be done by excessively intelligent individuals, like Connor and Chrysanthemum, the only two people who accomplished this feat. A similar reaction would have to do with the host's relationship with their Dreamscape. The nymph attaches itself to the consciousness of its host and slowly takes over during the times they are most vulnerable; while they are asleep. This is why Felix was able to directly communicate with his nymph, since he is a narcoleptic and much more experienced dreamer than most. Jace is the complete opposite in this regard, as he is a chronic insomniac. His nymph was slowly pushed out because it could not link to his dreamscape and has a non-tactile form in the physical world, however, Azazel is still permanently linked to his host.
One more interesting find with the injected nymphs is what occurs when they are added to dead bodies. Bodies no longer inhabited. The only three successful injections of this type are Cat, Birde, and Zeph. They are very strange in character and are completely without any memory of whoever inhabited their bodies before them. They often give off uncanny valley effect to everyone around them, (especially Birde), because they physically do not understand how to be people, and are eventually completely alienated because of this.