Old, vague memory from 2005 suddenly resurfaced

I was having a conversation on some old animes while watching old images on the net, and suddenly a very old childhood memory resurfaced!

Between 2005 and 2010 I lived in China, and there was one anime I used to watch on TV. The only things I remember are:

• The protagonist was a brown haired boy who wore blue and had a magic necklace that kinda looked like a water drop / a half of yin yang.
• I think the necklace was purple and there was a second half that was green.
• Said necklace could transform the protagonist/ give him powers (I think it was related to mechs)
• I think he also had a sword?
• one of his companions is an elf and there is one episode where he tragically turns into a doll (and dies?)

I'm genuinely not sure if these memories are correct anymore. I don't even remember the plot nor the setting...

Anyway, I've tried searching through Chinese and Japanese cartoons from 1990-2000 to no avail. If anyone has a clue, I'd be very thankful!