
Well.. for starters, they spent A LOT of time together in game, to the point where they had each others numbers and texted. (mainly Heejae annoying him but Seyoung still replies despite being in work so ??)
Also, Seyoung who supposedly "hates Heejae's guts" was probably the only one who showed genuine concern for Heejae (Heejae's appendix?? bursting and Seyoung calling an ambulance, which in turn saved his life.)
Keep in mind Heejae's a bit delulu and has had a grim childhood so having Seyoung who as youve pointed out, hasnt seen his face nor heard his voice, be concerned enough for someone they met online mustve triggered something in his brain. Seyoung's tsundere tendencies also played a huge part to keep Heejae's attention too lmao.
Personally, I think the manwha showed enough reasons why Heejae could possibly like Seyoung. I suggest you reread it if you'd want to fully understand why. (=・ω・=)

Wdym a lot?? From what I saw they only spent only a month excluding their time irl and that's it.
And if you're talking about the one that turned on Heejae even more is because of the "genuine concern" wdym by that? 'cause there's a literally a girl who spend time with him and she really likes him and I don't see any reason why he did'nt like her back if he really have a thing to the ones that have a "genuine concern" for him.
Plus he's a streamer which is sort of have an info or knows about what is happening in gaming world , I just see him as delulu person that really love giving things to the person he barely knows.
And I don't think so this manhwa had a plot hole in which I just said. Because he's literally a streamer so how can he fell in that ovb scam...

Theres no denying that Heejae is a delulu boy failure. Hes a certified gamer and chase and enjoyer. Its just that, as I see it.. Yoona(?) The girl, already lost as she came later than Seyoung.
She came at a time where Heejae was filled with anger/distrust/annoyance for Seyoung's "betrayal/escape." Hence why I think Heejae barely gave a shit about her and only talked w her superficially.
Yoona is, in my opinion, quite annoying w her approach to Heejae. She lures him out to stream/spend time w her through giving info about Seyoung. She also seems to only like Heejae for his skills and looks because come on.. Yoona just doesnt strike me as someone w a heart of gold who would invite out a man they havent met nor knew the face of, without any ulterior motives. (But if you think she is, perhaps we just have different views on things.)
I think at first, Yoona agreed to meeting Heejae, as theyre both streamers. She probably wanted to collab/stream together but faced w Heejae's looks.. Yoona fell(?.. not quite sure but assuming she did fall for him.) Yoona also hasnt shown any scenes w her being genuinely concerned for Heejae.. unlike Seyoung, Yoona hasnt saved him when his appendix bursted nor has she comforted Heejae.
About the giving things to a person online.. Heejae is most definitely a "bit" insane for giving Seyoung items that costs thousands, but I think it was shown in his POV that it was fueled by pride. Something about Seyoung thanking other guild members when they give him cheap items yet blatantly hate on him when he does anything near him. He wanted to show Seyoung that an item of this price doesnt matter to him as hes rich. So yeah‐ the gifting was just petty pride that was later fueled as he found Seyoung's reactions "cute".
As shown in his POV whenever he talks to Ohlala, Heejae knew that there was a possibility of Seyoung being a scam/netkama... but despite that, he still fell. Thats why hes a certified boy failure HAHAHHAHA Well, thats what I think at least... but who knows really.. ┑( ̄Д  ̄)┍
Ok but who fall for a gamer or player in game when you literally don't meet them?? I undertand that he likes him but he literally fell for him?? For what reason?? Like he did'nt even heard his voice...that's some horseshit I've ever seen