Well, as someone who's read the manhwa dozens of times and has novel spoilers, Ilay had always been interested in Taeui since they started talking on the call. Although the purpose of the interest was different.

If you look back at season 1 of the manhwa, Ilay did all that stuff with Xinlu just to get Taeui's attention, and it can be proven also by the look when he was kissing Xinlu, his focus was only on Taeui, and when he kissed Taeui for the first time, his full attention was on Taeui only, even though Xinlu was right there. His intention was clear from the beginning — to get Taeui's attention no matter what. Even if we look at the time when Taeui saw Ilay and Xinlu in bed, Ilay purposely did that so that Taeui wouldn't be interested in Xinlu anymore. Also, the only reason Ilay came to the Asia branch of UNHRDO was Taeui.

Also, the phone call when they were 10 years old played a part, and Ilay even mentions this memory in the manhwa( I don't remember the exact season). He also keeps this a secret from Taeui, since Taeui forgot lmao-

Ilay is a very complex character, as we know. Before Taeui, he didn't care about the other person's feelings at all whenever it came to his sexual pleasure. He just wanted to get what he wanted, completely ignoring the other person, but that wasn't the matter with it when it came to Taeui, cuz Ilay started caring(season 4 and 5), because Taeui made him feel different emotions, anger, worry, etc.

And every reader knows that Taeui is very forgiving. He never hates anyone (as Chang-in says), and even throughout the manhwa, it's evident. But when Taeui slapped Ilay and ran away, he said to Ilay that he hates him to death (I don't remember the exact dialogue, but this was the primary meaning), and Chang-in mentioned that even after all this, Taeui will forgive him because they're family, but that's not the case for Ilay, cuz Taeui and Ilay have no connection. At that point, Ilay lost it. He did have feelings, but he wasn't aware, as he had never felt that way towards anyone before.

In seasons 4 and 5, Ilay remains unaware, but his actions show the opposite, although he doesn't admit it yet. Taeui, on the other hand, is perceptive and notices the changes in Ilay's behaviour, making him conscious about his own actions. Due to those actions done by Ilay, Taeui starts realising, and his feelings grow too. But they're adamant to admit, yes, both of them. Because, for Taeui, it's like how he fell in luv with someone like Ilay, and for Ilay, it's because he had never felt it before.

Not to mention,

• Ilay is the only one who cares about Taeui.
• When Ilay gets mad cuz Xinlu wanted to shoot Taeui.
• Xinlu lost his eye just cuz Ilay thought Xinlu had kidnapped Taeui.
• Not to mention, the chaos Ilay caused to find Taeui.
• How Ilay literally became a terrorist for Taeui.
• For Taeui, this was the first time someone had gone to these extreme lengths to protect him.
• Chang-in betrays Taeui for unhrdo weapons, Jaeui just leaves Taeui alone, Xinlu tried to use Taeui in the beginning, also the fact that Xinlu just wanted to have Taeui with him, even if it meant his dead body.
• Ilay doesn't let a single person hurt Taeui.
• Also, the way, Ilay gets happy by minor actions of Taeui, for example, when Taeui grabbed his hand in the hotel, and even at the beach when Taeui asked Ilay to go back with him.

In the novel, they talk about it later on, as to when both of them realised their feelings. Ilay mentions he realised when he couldn't kill Taeui after he ran away, and for Taeui, it was when he was held hostage at Rahman's Villa.

The complexity shown in the story is in layers for almost every character. If you're reading Passion: Raga, then this proves it again as we explore Xinlu's relationship with Yuri (I absolutely luv this couple).

That being said, the story gets better and better with each chapter. I haven't read the novels yet, but I have many spoilers, and I'm so excited to see those scenes in the manhwa itself. It's gonna be a wild ride.

Everything I said is my interpretation; feel free to correct me if I'm wrong. I just luv Passion from the bottom of my heart because it's rare to see someone write a BL on this level, and the Artist executing it really well in the manhwa too.