In the novel, the romance was there but not too much that bothered the action. In here maybe it's not conveyed that smoothly that's why y'all complaining it's sudden and shouldn't be there.
First,this is the author's so it up to them what their character deserves for example here, some romance cause come on jin woo was fighting monsters everyday and can't that man feel some normal life aspects like having a good friend, relationship and not just about action (there's like 1-3 chapters that has romance for them so I don't see it as bothering the action so why are yay so pressed)
ANOTHER BIG SPOILER BE WARNED TO THOSE THAT HATE ROMANCE SO MUCH
HE AND CHA HAE IN ARE ENDGAME AND HAVE A FUCKING SON AND THE ENDING SHOWED JIN WOO AS A SOMEONE ROMANTIC CAUSE HE CELEBRATED THEIR WEDDING ANNIVERSARY JUST LIKE THE SCENE OF THEM FLYING ON KAISEL SO YEAH THAT'S IT A HAPPY, SATISFYING END FOR JIN WOO
I thought they were gonna be a couple but then the assistant also said to just kill Chris and that sank my ship ┗( T﹏T )┛