Vassal vs Vessel?

[Mad_Rat]

One thing that really confused me:: is the lady Faratras shoud be called "Vassal of Demon King"? I mean, vassal is someone who sworn loyalty to the overlord, and doesn't look like something she was doing. But if she contains Demon King body, she is the vessel, like, container?

Or am I missing something?

[Studio Daimon]

It's a play on the commonly known 'vessel', but it's actually called Vassal on purpose to recall the Vassalage towards a king or monarch which had mutual obligations. In this case, Faratras has to dedicate her whole life to (the cause of) the Demon King, and in exchange she's respected and revered for her hard work.
- Alfred

[Mad_Rat]

Well, from the prologue it looks like it''s the cause of actually containing the Demon King, right? It just sounds like she serves the Demon King, which is a bit confusing.