The G-imago is a small, slimy creature produced by adult "G". G-imago themselves can be created in either of two ways. A first-generation "G", such as William Birkin or Curtis Miller, will hunt down humans it believes to have the potential of carrying its offspring as hosts to the parasites. If the carrier is of the same blood-type as the implanter, they are destined to merge with the parasite and evolve into an adult "G" similar to the original. G-imago that are not compatible with the host's blood-type will instead be rejected within a few minutes of implantation and tear through the host's chest to escape. They will then live independent of the host and mutate into a G-adult.
G-imago can also be reproduced by these failed "G", but do not have the ability to mutate any further than their young state.
"G" that are either compatible with their parasites or injected themselves with the G-virus when human will evolve into this state. These adults have the potential of mutating any number of times in order to take advantage of situations. No G-adults created through blood type-compatibility have been witnessed.
Both first-generation "G"s William Birkin and Curtis Miller showed an impressive ability to evolve into forms deemed somewhat necessary. In 1998, Birkin would mutate several times in an attempt to use the environment around it to defeat Leon S. Kennedy and Claire Redfield, or simply escape - more agile or resilient forms, for example.
A problem with these adults is that their evolution is not entirely coordinated - William Birkin failed to re-evolve any lost traits and, after constant attempts to repair his body after battle, he was rendered a mere blob.
G-adults mutating from a G-imago incompatible with their hosts form into a somewhat-human appearance, with their back covered in tumour-like growths (some of which evolve eyes on for protective purposes). G-adults will not evolve any further, though are able to reproduce. These G-imago however, do not possess the ability to mutate at all. The clearest example is Monica in Resident Evil Outbreak.