He's a schizoid, if you really want to associate him a mental disorder.
To me, he seems to naturally have a carefree, detached personality. Maybe his power (Pika-Pika) played a part in emphasizing said traits even more cause most tasks are a joke to him... but from the few we saw of him he never had a leader mindset. He probably chose the Navy like you chose a job, he never had the drive to do it.
I bet that in a future flashback we'll see him being very gregarious towards Sakazuki & Kuzan since their very start of their Navy days. Always a step behind.