Comic by Sailor Scribbles.
Sailor Moon SuperS, ep 146
DIANA: That reminds me… the Queen and King of the future always had a fever just on the days they had stuffy ceremonies to attend. There’s really nothing to worry about because they’re always well by the time the event ends.
LUNA: The king as well?
LUNA: The king fakes an illness as well…
Hey, Sailor Moon fandom…
The star locket is mostly noticeable in the first chapter (the above) and chapter six (where he looks at it in the morning to check the time, and later it falls into Sailor Moon’s lap as he carries her away). Since he gives it to Usagi, he doesn’t seem to wear it again until they meet King Endymion in the 30th century, who has it on his waistcoat.
(Fun detail, in the “coronation” scene at the end of the Dream arc, Mamoru still isn’t wearing it when he briefly changes into King Endymion, suggesting that Usagi still has it. For more info about vintage tuxedo accessories, check here.)
But what about that neck badge? Another name for them are neck orders, a sign of someone either given an honour or being part of an order. According to Wikipedia, “most historic orders imply a membership in a group. In the case of most European orders, membership was also limited in number.” Chivalric orders in particular were fellowships of knights.
Theory and head canon: It was a symbol of Endymion as the prince and commander of the Shitennou. Act 6 and onwards, the neck badge gains a small crown above it, and funnily enough, that was after he kissed Sailor Moon at the ball. I’d like to think that was the cause of it “powering-up.”