I don't know how many times I Re-read Simoun and honestly, this manga holds a special place in my heart (with KZM and Blue Drop). Don't know why I didn't comment before and regret it but everything is perfect here, the univers, the story, the characters, the art etc.
I highly recommand the anime too.

The anime is a masterpiece and brought me to tears.

I watched the original anime back when it aired (...2006? God, I'm old), but I didn't know any of the manga have actually been translated...