When you hold a manga in your hands to read it, your thumbs hover at the edges of your vision while your eyes are focused on the page. But there are so many manga stories about people whose arms turn into monsters or machines that you might want to stop and check what your fingers are up to.
In the popular manga Parasyte, teenage protagonist Shinichi must come to grips with the fact that a shape-shifting space alien has taken over his right arm. A major character in The Kurosagi Corpse Delivery Service believes his arm to be inhabited by a space alien who speaks through an ever-present hand puppet. The title character of Vampire Hunter D has a mysterious entity living in his hand, with its face appearing in his palm. Relations between a person and a possessed limb are often antagonistic, and there is an episode in Jojo's Bizarre Adventure in which a demonically possessed arm actually tries to kill the man it is attached to.
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An interesting discussion why there is so many heroes in anime/manga stories have their arm missing or completely replaced. A humorous example from Midori no Hibi to more serious, gritty Gutts from Berserk.
I believe personally, it is the symbol of heroes willingness to fight for their beliefs, and they are hardcore enough to replace their own arms in expression of such determination. Gutts for revenge for example. When normal hands don't cut it anymore, a powerful replacement, an upgrade is in line to aid the hero in his quest. Interestingly, I haven't run across any main female character losing arms in this manner yet. Gender discrimination at work?
Cheers.
