In the origonal Devilman manga and Devilman:The Birth Welvath was a pretty young girl that Ryo Asuka had invited to his Black Sabbath party. Once Akira Fudo and Ryo have arrived at the party in Ryo's basement, Ryo goes on a rampage and attacks several partyers including the girl.
Once enough blood had been spilt the guests started to transform starting with Welvath she attacked Akira but he starts to transform into Devilman and he crushes her head with his hand, he then moves onto the other demons .
Whilst not confirmed a girl that looks a lot like the human form of Welvath was a member of Himura's gang and was the girlfreind of Masa . She is seen having sex with Masa in a minivan when a Demon kills Dosu-Roku and Meri-Ken Jou, she and Masa flee but are grabbed by Sirene and carried away she is killed and Masa dies a bit later.