At the very start of the reincarnation, you get a glimpse that this might actually be a tragic hero with complicated emotions. He states that he doesn't possess any senses, he can jam his fingers into his eye sockets and still see, and he's extremely weak. The problem is that rather then have him go through that emotional collapse since he lost nearly everything that makes him human, even more so than most of his other classmates, he brushes over it immediately. He becomes a one-dimensional character used to become a crutch for everyone else to stand on. The MC doesn't feel like an MC, his entire purpose is to make everyone else look good. And, that could be made interesting... except, he seriously has no problem with it.

I believe at one point, he even admits that he knew that the friend that betrayed him was always really just using him to make himself feel better, but he doesn't have a single reaction to it. I could be wrong, though since it's been a while.

Later on, we learn that he is just very empty and has no passions of his own, which is why he was turned into a skeleton. It seems their personality flaws from before reincarnation heavily influenced the monsters they were turned into. In order to become stronger, they have to face their flaws head-on and work to change themselves. This will be great for character development as the series continues.