When Mel first appears in "Terror of the Vervoids," she and the Sixth Doctor had been travelling together for some time, but in the Doctor's future so from his point of view, he is seeing an adventure he hasn't had yet with a girl he's never met. But no problem, this kind of thing often happens in the world of the Time Lords. At the end of Trial, the Sixth Doctor leaves with future Mel to take her to her own present after which he can return to his present where he will meet her past self for the first time. Or something like that.
If Mel's appearence is confusing, so is her departure, in the serial Dragonfire, she decides to travel on the Nosfaratu II with a two-bit criminal named Sabalom Glitz. Why? Maybe it had something to do with the fact that she had spent a lot more time with the doctor than he had with her. Or maybe not.
Mel was definitely a screamer. If there was an Olympics for screamers there would only be two events volume and pitch, like the broad jump and the high jump. Mel would be a shoo-in for the pitch gold medal. Every time she fires off one of her prime screams bats start falling out of the sky holding their big rubbery ears and going, "Ow, Ow, Ow, Ow."
Mel is played by Bonnie Langford, (born July 22, 1964) She was a child star, and after Dr Who seems to have done a lot of musicals.