Despite being powerless and believing himself to be someone who will betray ML and his party in the future, he's very kind, smart, brave, and principled. My appreciation went up really fast when he bravely protected one of ML's group member with his wit despite the potential of losing his limbs and even life. Also after the twist, I think he has became part of unforgettable characters in NU danmei xianxia.

They used to be together in the past. They had gone far, sooo far, that ML dared to be this unrestrained at the current time.

Not only that, Mengmeng also dared to do what he did because:

1. He had found out that Shixiong wanted to kill him despite their past affection and wanted to give him 'little' punishment
2. He was jealous and anxious. He wanted to check whether Shixiong had been together with his love rival
3. He couldn't restrain himself anymore because he had missing Shixiong for so long
4. But he was afraid Shixiong will hate him, that's why he can only play 'little' trick without Shixiong knowing

This doesn't completely justify his action. But for me it's enough explanation that what he did wasn't just cheap fanservice that I usually dislike.

Beside, if it was another yandere gong, they might have already ab*sed their lover because of all the misunderstanding.

Compared to them, Mengmeng was very obedient, cute, sunny, reliable but also a bit unreliable, romantic but skewed at time. But he did his best with his demon logic. I conclude this after seeing his overall personality including in flashback.

Author did write his first impression as yandere really strong, but I think it's just for impact and don't really represent his true personality.

You might have different opinion about him, but surely he is the type of ML that the more you know him the better he is (at least for the first 60ch which I think is enough to motivate anyone to finish the rest of the story)

There's never been a transmigrator. Xu Xinzhi has always been the original shixiong!!

He was trapped inside an illusion and lost his memory. I think there's really really tiny plotholes about this, like how it seemed as if Xu Xingzhi really came from different time and place, but probably it's just author playing us.

He was trapped inside the illusion by his other shidi, Jiu Zhideng, Mengmeng's love rival. He was pretty much still in love with shixiong, but it seemed Zhideng had never forced himself on shixiong so it's quite safe (NTR free?).

He was taken out from the illusion realm by another one of his old friend who also used to be friends with all of them (Bai in wheelchair something). It seemed he's now Zhideng's subordinate.

Apparently that Bai wanted to create a dogblood story where the amnesiac person killed his own lover without knowing.

But Jiu Zhideng didn't like dogblood feared Mengmeng would charm 'steal' Shixiong again and wanted to take shixiong back.

Anyway this really changes how we view things, like Xu Xingzhi's relationship with the other characters including Mengmeng.

I approve how fast their relationship change from a "transmigrator and their killing mission target" to "a pair who can do it day and night in all kind of places."

(They're finally together again in chapter 59)

Wen Bai Bao was the one who gave the dagger and the task of killing ML to MC.

Zhou Beinan sometimes can go through walls or touch objects, but sometimes can’t. Basically whether he’s corporeal or not depends on convenience. If he needs to catch something in order to save a life, then he suddenly has no physical body and can’t catch the thing; but other than that, he’s just living his regular old life as a regular dude.