Ready for this? Cap Harlock is a future Tetsuro. After gaining immortality, he regretted his immortal quest and hitched a ride on the time castle in order to subtly influence his younger self to seek out humanity and savor mortality.
I would also argue that Mattel is indeed Tetsuro's mother having regained her life at the hands of Harlock in exchange for his mortality. Harlock was tricked my Prometheum into reviving his mother rather than his true love Maya and so Harlock becomes Faust (yes, he is his own father i.e. he loves Maetel/Maya). Now they are both cursed in time to live out immortal lives of regret and incest. The young Tetsuro supersedes the fate of Harlock and avoids doinking his own mother.
I know it's a hell of a concept to read into, but I like it and I like to think Leiji would too.
Review of Galaxy Express 999
