Narcissistic Personality Disorder: Marked by an exagerrated sense of self importance, self centeredness, an exploitive attitude, and lack of empathy.
Janus does show some sense of self importance and self centeredness with his arrogant ways as a prince, does he not? He does have a lack of empathy which has gone on throughout his entire life, and that's telling us something too.
Obsessive Compulsive Disorder: Persistant, recurring, involuntary thoughts (obsessions) or behaviors (compulsions), or both.
Janus' vengeance against Lavos plays a key factor role in Obsessive Compulsive Disorder. Janus is obviously peristant in avenging his sister and wanting to kill Lavos. This leads me to also believe that he may have persistant, recurring, and involuntary thoughts that lead to involuntary behaviors that play a role in his desire for revenge. All his life he's wanted this, do you think Janus is going to stop after losing everything but the clothes on his back and his own life? I think not.
Major Depressive Disorder: Refers to overwhelming feelings of sadness, despair, worthlessness, and hopelessness, and in extreme cases, suicide.
I'm sure that Janus has had this happen. I'm sure that he didn't start out all gung-ho on killing Lavos and shit. I bet when he started out surviving that he was having overwhelming feelings of sadness and despair over the loss of his sister and everything that he had. He may have felt overwhelming feelings of worthlessness and hopelessness in allowing such an event to happen and such a thing to happen to his sister, and might have thought things such as, "If only I had done this.." But I'm not sure, but it would make sense in my opinion. I doubt Janus was suicidal though since he was obviously hellbent on revenge and finding his sister. But, I'm sure there may have been times that he thought of ending it all...you never know.
Dystymia: A milder form of depression than Major Depressive Disorder; none the less is chronic.
If the above disorder does not fit Janus as severe, this would, in my opinion.
Disorganized Schizophrenia: Is marked by extreme social withdrawel, hallucinations, delusions, and bizarre behavior.
I don't know if Janus had hallucinations or delusions but it would seem obvious that he was extremely socially withdrawn and I doubt that some of his behavior was seen as "normal" and may have been considered bizarre, but I do not truly know.
My opinion; if Janus doesn't privately and publicly express all the symptoms of these disorders, he at least expresses one or more symptoms of each said disorders including Asperger's Syndrome. And as I have stated before, whether he has knowledge of his disorders or not, I would
highly doubt that he would ever seek help or even pay the disorders a second thought.
This is all just in my opinion and from my inferences on the matter.