So umm... Pokémon Masters is a free to play Pokémon game (with in app purchases) available as an app on both android and iOS devices.

Basically, when you start the game, you would be able to choose your character:

(There is more customization in game, including hair and eye color and skin tone)

But the thing is, in the entire game, you and Red are the only two sync pairs that do not speak.

What makes it even stranger is that the other people in the game would still know what you are saying, even though you don't actually say anything. This is so as you can choose what you want to say at times. However, when you use yourself in actual combat, you don't actually make a sound at all... while all the other sync pairs you've got would say something.

The same goes for Red, apparently, as Blue is able to tell what he wants even though he's speech bubble is literally full of dots (reference to Professor Oak's Sync Pair Story)

Like seriously, Red speaks dots but Blue is still able to understand what he's saying!

But, seeing that all the other sync pairs could talk, this 'muteness' seems unique, one of a kind even, and maybe even something that could only be passed on through genetics.

So, I did a bit of research and found out that even though some kinds of speech disorders are inherited, they are not the kind that would render one speechless. (Besides the one that makes people afraid to speak but you can tell that your character communicates well with others in the Sync pair stories.)

But still, there's a way to save this theory.

And that makes this entire theory even crazier.

Theory: You and Red are not actually mute.

In the midst of doing my research, I found out that there is a kind of disorder that may fit into the missing piece of this jigsaw puzzle.

This disorder is called the Developmental Verbal Dyspraxia, which makes it hard for people, especially children, to speak clearly.

So, what if the dots are not dots but rather Red just speaking in an unclear manner?

This would explain why you make no sound when you're in a battle. You're choosing not to make a sound because of your disorder. However, people could still understand what you're saying because you can still talk, just not clearly.

God, I'm making all of you rethink Pokémon right now. Aren't I?

Oh and more evidence:

'the two of them' refers to Red and Blue, as they have just walked off the screen. Could this be professor Oak hinting at your relationship with Red?

Well, who knows?

By the way this theory is most probably junk (I think I can find many things wrong with it already) however, it's my best explanation for this phenomenon.

Oh well...