Mephisto, "The Singer" is likely a reference to the Caladrius in Roman mythology.
Before being confirmed to be a Liberi from his transformation to "The Singer" in Episode 08, Mephisto's race was a subject of debate among Arknights players, due to his fully humanoid appearance with an apparent lack of animal features.
The feathers he released when attacking and defeated made Global players (correctly) speculate that Mephisto is a Liberi.
Some CN players initially speculated that Mephisto is a Sankta who had lost his wings and halo.
The Singer's DEF and RES' rating stats are inaccurate, rating higher than the factual numbers which could have been more closer to B rating.