Мейтель

The enigmatic woman who gives the young boy Tetsurou his ticket on the Galaxy Express 999. As beautiful as she is wise, Maetel provides constant support and guidance for Tetsurou throughout his travels. In Leiji Matsumoto's own words: Maetel "is the woman who travels with an adolescent boy's dreams. She's traveled with millions, tens of millions of boys, so far. In fact, there are as many Maetels as there are boys. That's also how she can show up in any story. All boys have met Maetel in different forms of encounter. That goes for Antares or Emeraldas or Harlock, as well. They are all crossed over, and they can all be separate stories on their own.

So if you change perspectives, you can make as many stories as you like. The characters of Shadow and Emeraldas, who were girls in their adolescence, have met someone called Maetel, in the heart, from their childhood, or during the time when they were full of dreams. It's a train ride through one's mind, or the train that travels along with one. And each person has a different journey..."

In chapter 68 it is said that she is always dressed in black because in mourning for her lost companions.

(Source: Anime Interviews: The First Five Years of Animerica, Anime & Manga Monthly, First Printing in 1997)