sH9

First, Taejoo the ML is definitely a red flag - r*pe, debt, isolating the MC etc - and second, he clearly fell in love with Euihyun the MC and feels guilty towards him. BUT, that does not mean he will be good to the MC ! Indeed, he hasn't addressed many of his red flag aspects:
- He is possessive, to the extent that he'd probably refuse to let MC have a healthy love life with someone else ;
- He doesn't want to be away from MC to the point of probably refusing to erase his debt which means to let go of the only thing that ties them together, and also refusing to let him abort (if MC is pregnant like was hinted at) ;
- He is brutal and ruthless, although he doesn't tend to be physically violent towards MC (except for chap1 and the heat scene) he is very willing to hurt other people, physically or otherwise, which - paired with his possessiveness - means he could hurt people around MC ;
- He lacks communication and basic common sense, like I personally believe he didn't even realise the MC was going through so much anxiety during the period he rarely visited him, I'd expect that aspect to make some progress but nothing can be certain, since he may both be oblivious, be quick to assume things and lack the capacity to create opportunities for both safe and frequent genuine communication - but, that aspect may be the one in which the quickest progress could happen.
All that to say, that when a RED FLAG harmful person, even with some light in them, starts to want good things for their loved one and wants to be better to them, that doesn't automatically make them a green flag, or even a yellow flag (if that expression exist).

(⊙…⊙ ) I must say... the character design here is very good
it shows a lot in the various shots in "Special. : Author's twitter + fanarts"

Even though I personally didn't enjoy the webtoon, I'm so positively surprised by the quality of the design, how unique and beautiful it is for all of the four characters presented!