Last words

Naikendan: I'm promising you right now, if we do survive, it wont be cause I'll have you dead!
Kaichi: Please... can we not be fighting during our last moments together as family? Can you forgive me as my last request...
Naikendan: Only if you can complete my last request.
Kaichi: OK, what is it?
Naikendan: For you to get us out of here!
Kaichi: I hate you.
Naikenda: I love you too.
(this conversation happened before they had the rope in there mouth so they couldn't speak )

.........................

Anyway this was way before Nai was even a vampire, a spoiler for Nai's past I guess but I dont care
Can anyone guess what exactly Kaichi is to Naikendan?
His family but what is he in the family (cousin, brother, twin, dad, etc,)?
I give free hug to who ever guesses
Also for anyone who does critique keep in mind it's only supposed to be sketchy.