In Galaxy Express 999 (the movie) was the purpose of Tetsuro's pendant ever explained? We see at the end that Maetel was holding one that embodied her dad's soul and it is what she uses to destroy planet Maetel, but Teturo's is never really explained. Or did I miss something?
It's not really explained, but it's known to be a picture of his mother Kanae. She gave him the locket before she died, which he keeps as a memento.
It may seem strange that she would have a picture of herself in a locket, but in the second movie, Adieu Galaxy Express 999, it is revealed that Faust (implied to be Tetsuro's father) has a second identical locket with a picture of Kanae and a young Tetsuro, which he leaves to Harlock. So it seems that they were a set, perhaps exchanged by Tetsuro's parents.