Yeah, the biggest reason is that Hajin didn't want to get close to anyone while he focused on completing the story and returning home. However, once he sees that they are more than just characters but actual people, he does start interacting with others better. In addition, his actual love story will start later on once he works full time with the Chameleon Troupe.
wait, gimme a spoiler
oh i see! that gives me a better understanding of the story's flow and hajin's potential romance
Spoiler:
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Hajin will be paired with the Troupe's Leader. After assassinating Nayoon's brother, he will leave school and work full time for the Troupe. He also alternates between his home with Evangel and the Troupe's base, and redecorates their base since their base has this undiscovered evil energy that makes people go insane.
for once, or mybe twice idk, i cannot sense the (mutual) romance in the story. honestly i can see more potential with yeoryong but im not feeling the others much ┑(￣Д ￣)┍ i think its bc the lack of reciprocation with hajin's reactions via face and thinking bubbles (except that one chapter with nayoon, but that feels a lil more sudden with his personality) IDK I CANT DESCRIBE IT PROPERLY
i get why nayoon and rachel would admire hajin tho! but i think this story would be more fun if hajin didn't end up with anyone at all and just continue raising Evangel on his own. (purely an opinion) i dont mind romance in shonen mangas really, i think it adds more spice within the story—its just this one time that i cannot fully "feel" the potential /mutual/ love story with hajin even though I've read this.manhwa from start to recent uprate STRAIGHT
maybe if i reread i could perhaps find more clues