I love the ending where it explain that sooin's blood is like poison because it can make a vampire back to be human that's why chi-hwan can drink it because it also says that a vampire with a link can't drink anyone's blood except for it's link. And now the bastard who harassed our sooin is a vampire and currently dead (thanks God) his blood is still pure vampire so chi-hwan dear used his blood to sooin become a vampire to survive then when our sooin become a vampire he sees chi-hwan dear in the verge of dying and he can smell its blood now (I think it's smell like a human blood now) used his blood (sooin) to save chi-hwan and then they become link since I think that's how they do when making a link to a human.
That's all hope it helps (づ ̄ ³ ̄)づ
So for everyone who's confused till now which is which here's a little explanation on what happen
Chi-hwan = becomes human and link of our sooin
Sooin = becomes vampire because of the bastard vampire blood and made Chi-hwan his link
Sooin blood (when he's still human) is a kind of a pure blooded human so I think it can make the vampire who drink it slowly turned it back human (that's what happen to our chi-hwan dear here)
Ps. Comment below if you're still confused
Pps. Chi-hwan dear lookin so hot under Sooin like (~ ̄³ ̄)~
Wait so chi-hwan became human-
Yep yep yep
BESTIE THANK YOUUU FOR EXPLAINING MWAH