I dislike him too. I remember Yuyang in Guang Xiang was faced with the same situation, but instead of lying my lovely Yuyang never failed me 'coz he actually helped the MC to realize his true feelings, which Jang-Gyun denied to Jimin. He's a spineless guy who only grows a spine once the situation is favorable or manipulated to his favor, so petty.
I don't think he's spineless at all. If anything, he's the most decisive character. He lied because he wanted to put Jimin at ease and preserve their relationship. Then, at the cafe, he started to deny Jimin's feelings for Hyunwoo, but he realized that's not what he should do because it would be horrible for both Jimin and him, as it would effectively deny his own feelings for Jimin. So he decided to come completely clean about everything. He had the resolve tu stay true to his feelings in the end, and for years before that he had the resolve to do everything to preserve their friendship. He hadn't manipulated any situation into his favor, nor has any situation even been in his favor at all.
I don't mind the room only because I don't think he's a scary or stalker-y personality, and because he specifically told Jimin to stay away and he ignored his wishes. I agree though that he didn't need to lie; considering Jimin's reaction anyway, he should have come clean when he went to his house, cause it's not like his lying put Jimin at ease anyhow.
What I meant by "spineless" is the inability to be truthful and not indecisiveness. Also, he did not completely come clean about everything, because he has not corrected his lie concerning Jimin's true emotion, which he is clearly shown to manipulate and twist the interpretation thereof. This situation is clearly favorable to him, because when he discovered that Jimin has fallen in love with Hyunwoo, he consequently confessed his feelings.
It's so simple that he is treading into "manipulation lala-land" albeit lightly and slowly, so I don't know and care why many are still blindsided by his looks and 1st impression. It goes along the line of "gay seme does not confess to uke, because he is uncertain if uke is straight or not, but once he discovers that uke is gay he lied to his advantage and confessed his feelings that were pitifully hidden for years." IMHO, none of his actions will help his defense to me, 'coz he may not be a yandere psycho yet but that does not mean he is still right and correct.
Reminder: "Lying can never be a good deed."
Personally, I'm disgusted by the way he shows his love for Jimin. Dedicating an entire room to pictures full of him, yelling at him and being posessive about where Jimin goes and who he meets, and even considering to lie to him to benefit himself when Jimin decided to put his trust into him and go to him for help are things that not okay, even if he is apologetic about it. He's crossed the line of not-okay things a very long time ago. In fact, I was surprised Jimin still wants to he friends with him. While yes, I do want Jang-Gyun to fall out of his posessive love over Jimin and find somebody who reciprocates his feelings, no way is Jang-Gyun and Jimin's relationship ever going to be seen as healthy to me.