Yay! Mettaton! My favorite character in Undertale!! I don't have a life-changing theory, but something to think about. Here's what we will cover in this part:
>>His heart
>>Gender
Let's go over what we already know about Mettaton. He is the cousin of Napstablook and used to live with them before he became a star. ((Btw, as much as I believe Napstablook is a dude, the game refers to him as 'they/them' so I'm going to try to keep this part as accurate as possible.)) Mad Dummy or the training dummy at the beginning of the game could also be kin, but this isn't confirmed. We know that he's a ghost possessing a robot, and that he's fabulous. Ok? Ok. Let's get started.
I'm going to start with something that some people are trying to change and make theories about: Mettaton's gender. We don't need to go deep in mystery or speculation to confirm this one. The game refers unspecified monsters as they/them to point out that their gender is unknown/undetermined/neither. However, Mettaton is referred to by all the characters as he/his/him. So, he didn't change his gender after transforming to EX; he just looks kinda gay.
Now let's go on to his heart on the belt... I've noticed things as I played on in the game. Have you noticed that when you kill Mettaton in Genocide or Neutral that his heart/soul doesn't turn upside down and shatter like all the other boss monsters? Is the heart his soul or the powerhouse? This little heart reminds me of the human souls in the coffins in Asgore's castle. Each one has a specific color that goes with a specific attribute. Red is determination, and I'm pretty sure yellow is justice and light blue is patience. However, there's no pink soul.
I'm not saying Mettaton is a human; there's not enough evidence for that. His heart/soul isn't white or pale grey like all the other monsters, though. So... My theory is that he represents an attribute similiar to the seven human souls. But what could it be?
Well, it's really noticeable that Mett seems full of himself and maybe even a little narcissistic, but my favorite trait about him that he has this 'follow your dreams' and 'reach for the stars' type of aura. We can see this in the diaries in his house in Waterfall (the red house beside Napstablook's) when he says he chose his quest for stardom over his own kin. He seems so set on becoming a star... So I would think he represents either confidence or faith.
Also, in the lab, there is this machine with rosy pink stuff coming from a drain... Could this be connected to Mett and his soul, or just one of Alphys' many experiments?
But wait! This may not actually be his soul. It could probably be the powerhouse of his robotic body painted pink, his favorite color. But keep in mind that this game had a lot of thought put into it, and I'm pretty sure Toby Fox nor the others would add something specific and eye-catching on accident.
What do you guys think?