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They'll get separated later and even when they still share a body they can communicate just fine like they did now in the manhwa, in the novel hoyeol telling grandfell to be a good boy or sometimes calling him "my grandfell" and then there's granfell rampaging when hoyeol went unconscious for a bit he said "you've stolen a ray of light from me" blah blah and even called him "my dear", and then in the future hoyeol also brought granfell to meet his family and even chained him, there's also their confession lines, the way both of them thought of hiel as their child...
There's so many moments about them so if you want detailed plot and fruity bromance I recommend you read the novel too
To continue from the manhwa you can start on around chapter 150-153 but since many details got cut off and scenes were changed in the manhwa i suggest you read from the beginning instead, here's the link:
https://pindangscans.com
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If you're talking about his long black hair then it's right after grandfell left(only last a few chapters tho), he was really depressed when that happened and ended up dyeing his hair silver to began pretending to be grandfell. He continue pretending to be grandfell till now(latest chapter novel) and only showing his real self/personality to grandfell...

I heard this was gay as fuck someone confirm this