main couple:
lucaon x kyon, lucaon hated humans and didn't want to be associated with any because they fed his parents and brother vs solution which made them turned into zero (evil being like zombies). kyon liked lucaon since the beginning because lucaon once rescued him from a zero attack. their relationship slowly changes.
second couple:
sihan x karis. smooth sailing not much drama. just that sihan is over protective for karis. karis felt offended because sihan said and treated him weak.
third couple:
saddest story. ryan x christoph. christoph had his memories erased by his half brother which was voltron (group which uses vs solution to kill royal bloods) and sent to ryan to gain trust and kill lucaon. he was forced to obey voltron and couldn't resist the attack. ryan died in the end and christoph's memories of ryan got erased. no happy ending for this couple TT
Royal Servant