Mettaton cares for his cousin Mad Dummy, whom he keeps hidden away in fear that he may end up like their other cousin Napstablook. He himself suffers from Narcissistic Personality Disorder (NPD), but with how flamboyant he normally is it's hard to tell.
In MentalTale, Mettaton's body is no longer pink, but is a dark green. He also sports a bright yellow eye, much like the color of Undyne's. Mettaton credits Alphys with this, but admits that he went along with the changes to appease her. Even so, he still seems to take pride in his appearance.
Mettaton's personality is almost identical to his Undertale counterpart's. However, despite being so flamboyant, he's actually very mellow when around his friends and family.
Mettaton has a great amount of respect for Alphys, and visits her often.