The species that Saya is a member of are extra-dimensional beings.
Saya's species are very resilient but not invulnerable, being able to survive multiple gunshots and other blunt force trauma attacks. However, severe damage such as having a large part of their bodies destroyed or repeated blunt force blows will still kill them.
Their resilience comes with what their body is described as by Dr. Masahiko Ogai's journal, as being fleshly, like sponges or fabric that flexes and expands in all directions which they can control to allow themselves to move quickly and precisely. This allows them to expend enough force to break a human's neck and rend people open with ease. Their body's trait also allow them to cushion and absorb attacks that would cripple most humans.
Saya's species are capable of modifying the bodies and flesh of any species they have samples of genetic material from which they must ingest to absorb the information. This power can get down to modifying a creature's actual genetic code or perhaps their entire ability's catalyst is based on their power to manipulate the genetic code. If the species desires, they can completely unweave a creature's body, cells, and genetic code and change it into something else. Large amounts are needed for them to fully understand complex organisms, so it takes them some time for them to perfect these abilities. Though this did not prevent them from experimenting on victims to test how far their abilities had developed.
The species' greatest power is the ability to 'bloom' and envelop an entire world with 'spores'. These spores begin changing all life of that planet into the species. They do this similarly to their ability to change the bodies and genetic code of living beings, but on a global scale. The spores act like a virus and change the genetic code of creature to mold it into one of Saya's species. To get to this point, they must go through a long and lengthy process of ingesting large volumes of genetic material from eating and absorbing many various creatures from a given world. Saya's species is then pre-programmed to expel themselves and 'bloom' once their body has processed enough of that planet's various creatures. It is unclear whether or not this ability kills the being after they complete this process.
These creatures posses unimaginable intelligence, with Saya solving many mathematical problems that no human or even computer had been able to solve.