Spoiler Majora's Mask: Termina and Hyrule Differences and Similarities and More.

[Beauts]

I think (well in the case of Link, I know) that Link, the Happy Mask Salesman and Skull Kid are the same in OoT and MM. All other characters are unimportant counterparts just to give an eerieness to Termina.**

I think the Postman is included in the latter category because you barely see him in OoT except when you get the bunnyhood or do the unwinnable race (I always thought they were the same guy just seven years difference). In the theory that Link is dreaming of Termina, it would indicate that he is reusing faces he has seen throughout his childhood in his dream and giving them alternate personalities. This explains Koume and Kotake, for example.

**Link is wandering the Lost Woods at the start of MM. Skull Kid lived in the Lost Woods, and was always trying to make friends. This makes him basically the same in both games. As for the Happy Mask salesman, he basically tells you he travels looking for masks.

 

[PapilioTempesta]

Hang on, doesn't Link meet Koume and Kotake when he's an adult? How does he remember them, if he's dreaming Termina? (Or he does have memories from his adult adventures? I still find this parallel timelines confusing)

 

[Red_Link 64]

It is quite possible that there are some Terminan perople who do not have Hylian counterparts. But, the Hylian counterparts may have also died, got physically changed, ect. It is said (Or at least practically said) that the Salesman in MM IS the same as in OoT. If I remember right, Anju (The Redhead) has blue hair in OOT so Kafie may be some Redhead dude in OOT. As for the girl and Redead dad, they are, like the salesman, very unusual. The fact that there are people brave enough, besides Sakon (the theif) to live in Ikana Valley may suggest something. But this may just be me being paranoid.
To answer the qeustion of how he remembers Twinrova (Koume and Katoka), he is sent back by Princess Zelda in the future to warn Princess Zelda in the past so it is safe to say he knows the future that won't happen in the child timeline. Or else he couldn't warn Zelda.