I can't help to think the Ranzikan is the eager side of the Cadgan, (or the demon had made deal with him first to suck out him passion first and the demon is now using the emotion)and they will merge into one to become the familiar Cadgan? Sorry just my daydreaming, But the mark and the color of their face are so like opposite that makes me wondering
(SPOILERS mentioned)
I had a similar thought about Cadgan maybe having made a deal with Ranzkai as a child to cure his sleep disorder, Cadgan's side of the deal being to do with his feelings for Hector. Since it was like the day after that Cadgan and Hector shared a sweet moment that Cadgan no longer had any sleeping issues. (edit:)Cadgan also doesn't seem to have any real issues going into the valley unlike everyone else which I would think hints towards a deal.
Not sure about them merging or anything but guess we will find out!