What was the meaning behind Tetsuro's pendant?
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.