Male Main Character is Hiding Something

He's hiding the fact that he used to be a bodyguard.

Blades!! From his body!! What?

He can see dead people.

He became a doctor and got a job at a prison so he could kill the man who killed his father. Oh, and the guy at the hospital is his adoptive father.

Half-way through, his PTSD really starts to manifest . . . he tries to hide it.

He's living a double life pretending to be his elder brother (who is currently MIA) because despite being ten years apart in age, they could be mistaken for twins. So, he's running back and forth between high school and a big company where he's a Director.

He can read minds. Oh, and he also hides who he is from the female love interest (they met when he was little and she was in high school).

He's hiding his major social anxiety, as well as the truth about what happened to the female love interest's older sister and how she died. Oh, and at the beginning I think he hid his face as well.

He has an actual, honest-to-god, human allergy (the cause is psychosomatic, but still). If it gets bad enough, it can actually be fatal. Most people besides his doctor don't know about it.

Main character is a Nation Intelligence Service agent. This is a secret. (His identity is known to only a few high up people).

He hides what happened to his parents and how he ended up living with the female love interest, and he keeps his real name a secret. FYI, his parents are both dead and his brother is MIA.

Main character and his best friend are hiding the fact that they know each other from over ten years ago. Plus . . . Je Hyeok just seems to hide things in general. I wouldn't say he's secretive, so much as . . . he omits things? Neglects to mention them? He's a man of few words, let's put it at that.

Main character has Panic Disorder, and doesn't do well with crowds or public speaking.

He's . . . he's a vampire. 

Supposedly a brain injury caused him to be unable to emphasize or feel emotions. This happened when he was very young.

Main character lost his hearing, and at the same time witnessed his beloved grandfather's death at the hands of his step-dad.

His mother forces him to learn to read lips and takes all sorts of measures to hide the fact that he's Deaf.

Main character has Dissociative Identity Disorder. (Previously known as Multiple Personality Disorder, or Split Personality).

Main character has Dissociative Identity Disorder. (Previously known as Multiple Personality Disorder, or Split Personality).

His father's wife tried to kill him, his mother, and his little brother after his father died, by having someone drown them. Him and his mom survived, but she later died as well, and his adoptive father, too. He spends around twenty years working to get revenge, all while hiding his identity, and living with the very same people he plans to get revenge on.

Main character is losing his hearing. 

An aliens parasyte has taken over—or should I say replaced?—his left arm. It’s given him . . . abilities. Strength. He’s changing. 

Son of a Yakuza boss goes back and attends high school so he can finally graduate. The yakuza bit is a secret, as well as the fact that he’s about ten years older than the rest of his peers. 

Main character and his ex-best friend are very secretive about their relationship and their past. MC also hides things like the fact that he’s a year older than the rest of his classmates, and he used to be a real delinquent and really knows how to fight.