Comment by DW047 on Childhood Friend of the Zenith - Chapter 87

Iirc, the one who resides inside that meteor sword guy is the Blood demon and he's stronger or on par with MC's father. He wasn't completely sealed away by Elder Shin back then they just sealed/took his powers away from him and that powers created both Cheonma, Seol-Ah and the dancer making them sisters. And if he consume the three of them he'll get his full power back. He knows about the regression too but he didn't know who regressed and he didn't retain his memories from his past life.

the one the MC is fighting now. And the Dancer is the blood Demon's most trusted aid. She's strong but not as strong as Seol-Ah and Cheonma. She's the weakest of the three sisters that's why instead of going against their father (blood demon) she serves him because of jealousy to both Cheonma and Seola-Ah.Maybe she's hoping the Blood demon will make her stronger not knowing that she'll be consumed eventually.