The M-series refers to a group of modified humans who were experimented on by Dr. Crombel. They are described as "failed" experiments, due to the majority of them being unstable and seemingly weaker than the more recent experiments. Only two M-series experiments have been introduced in the series. And it has been stated that they can't remember their original names.
They were 100 humans, who were experimented on for human body enhancement by the Union. These experiments were led by Dr. Crombel. It is speculated that the experiment for physical modification was at its very early phase. All of them started dying one after another due to the intensity and cruelty of the experiments. M-21 and M-24 were also supposed to be disposed of, but due to whims of higher-ups from The Union they were placed as low-ranking agents of the organisation. Only one of them, M-21, is currently alive.
Powers & Abilities
Details about the experiments were hidden and so far only Dr. Crombel knows about the full details of the experiment. Even Tao could not find any data on the M-series.
The only powers known are of M-21 and M-24. Other than enhanced strength and speed, M-21 has a werewolf heart transplanted inside of him, which gives his similar abilities including even faster speeds, physical transformation and high speed regeneration. Whose heart does M-21 possesses still remains a mystery.
M-24 had a very high physical strength and endurance. In addition to that he had Mind Control and was able to create Infected with intellect unlike other infected, which usually lose all sanity. This ability is similar to nobles and hints that the experiments were carried on to create modified humans with noble-like powers (with the exception of M-21).
Since the M-Series has characteristics of both Werewolves and Nobles, Dr Crombel may have been trying to produce a hybrid of the two species. From M-21's memories of his M-100 Comrades they may have had a telepathic link with one another.