HELP! If you've seen last episode of TV-series or understand Japanese!

I know this post was made almost a year ago so maybe you already know, but here's what I understand from watching the clip. I hope it helps!

The concept especially in this anime version, is that Maetel is a "illusion of youth". The narration at the end explores this idea.

Maetel helped Tetsuro grow as a person, and now that he has clearly stated to the train conductor that he will be returning to earth to make it a better place, she feels that it's time for tetsuro to go on with his own individual life by himself. And she says in the letter, that now she will be traveling with another boy, just like tetsuro, and support him to find his way, and that she will eternally keep on doing this, going from one trip to another.

So to answer your question directly, the boy in the shadows is not someone specifically identified, but he was made to resemble tetsuro to illustrate the continuity of maetel's never ending trip. Sad ending! But a little different from the movie where she says she will go retrieve her original body.