Dr. Mario, often referred to as 'Doc' by Mario and his friends, is an antagonist in the series and plays the role as Mario's doppelganger and rival. While he doesn't hold as much of a threat as Toad or Gree, he does cause conflict and annoyance to Mario throughout the series from time to time. He allies with Dark Pit during SSB4 to form a stronger alliance against Mario and his friends.
Dr. Mario is very easily smarter than Mario, and while he doesn't take Honors courses, he is manipulative and scores a lot higher on tests. He is seen as a bully by some people, and his hobbies consist of torturing animals for his own pleasure. Most people try to ignore him in school since he is overall very scary to be around, but he becomes intimidating when rumors begin to spread around the school about how he is stronger than Mario. He gains power through blackmailing and force, which gives him a small boost in authority around Nintendo.
Strangely since his age is the same as Mario, Dr. Mario has a grown out mustache. Some think it's a fake for him to look like he's older than others, but it also may be because the creators were to lazy to edit it out of his picture.