It's like a combination of Inception and The Prestige.

No matter how one looks at it, MC is well-loved by many people around him throughout the plot. Not just bluff, he is a person deserving of all that affection.

Xiao Yan is brave, smart, and chivalrous. He can risk his safety just to help people he only met once or twice like Maya, who almost takes advantage of him. He is willing to sacrifice himself to protect Lily, whom he likes, though she turns him down for another man. He almost got himself killed many times, just so that he could save more people or not let The Surge organization use him for bad deeds. He rarely ever holds a grudge and is compassionate toward people around him. He has his faith, knows what he needs and diligently pursues them.

He is a bright light in this dark, fallen world.

ML, on the other hand, was feared and respected by many. People trusted him, but no one dared to love him.

Different from others in his army group, he determined to distance himself from others, no dating, no hooking, no 419, nothing. Because he knows exactly that he won't live long, he doesn't want to make anyone sad about his death. Because he hates betraying and loathes traitors.

In the past, ML was not an indifferent person like who he is now. He used to be just like MC, a promising student.

But...

After a shocking twist in the past when his mentor becomes a betrayer, Heine abandoned the peaceful life of being a researcher, training himself to become a soldier, who faces danger every day for the interest of the base. He never regretted that decision and had made up his mind about the unavoidable outcome (death) of it. Meeting MC makes ML, for the first time after becoming an elite force soldier, want to stay close, want to love someone dearly. ML's life used to have only the darkness of killing, losing, and betraying, but MC has shone a light on that darkness and changed ML's mindset.

Though, his life is coming to an end soon... He might die any time. That is the cost of becoming an elite soldier of the special task force.

ML desires MC more than anyone else, but unlike Jane or Maya, who sexually harassed MC right on the spot despite MC's rejection, ML holds himself back and uses a milder approach, which, well, effectively makes MC fall for him and motivates MC to grow and create. He wants MC to become stronger, for MC to have the power to do freely what he wishes for without relying on others. Even if he was not here.

2nd ML Jane is likable but he is simply too reckless, selfish, and crazy if compared to ML.

There is hardly any other that can change MC's mind but ML. Because of ML's encouragement and support, MC has become a better person (From a weak B-rank student to a versatile, strong soldier, researcher, and pioneer in only two years). That is something not even MC's closest friend Casey could have done (motivate MC) after many years. But ML has done it successfully.

And if MC doesn't like ML enough, he would never desperately research the viruses and create the artificial X-2 virus, which saves ML's life on time.

I believe that even before the second mission, the one with the traitor Manson, ML has already interested in MC. And after the 2nd mission, ML definitely liked MC.