I fight in the battle between Darkness and Light.
My owner is the sky.
I have one shape, but I am two things at the same time.
If you miss your body or your heart, be afraid of me.
Who am I?
My answer is
A keyblade from Kingdom Hearts
I fight in the battle between darkness and light
The whole game is essentially this. Literally forces of light vs forces of darkness
My owner is the sky
The main user of the keyblade is Sora, who's name translates to sky.
I have one shape, but I am two things at the same time
Its both a Key and a sword? I was unsure of this one.
If you miss your body or your heart, be afraid of me
Heartless and Nobodies, both missing either body or heart, are common enemies.
My answer is
Death
I fight in the battle between darkness and light
Death is grey, it's not bad nor is it good.
My owner is the sky
Death could be considered God who we usually believe to live in the sky.
I have one shape, but I am two things at the same time
Death could be life too
If you miss your body or your heart, be afraid of me
if you're missing one of the two you usually feel afraid and are going to die.
Considering games tag, maybe you're a:
War (form Darksiders). This also could be any of 3 others horsemen (Death which was mentioned before as an answer, but not exactly in that way. Or Strife or Fury).
Cause:
I fight in the battle between Darkness and Light.
Well, the whole plot is about heaven and hell divided and you're fighting sort of between them.
My owner is the sky.
basically all 4 horseman came from the 'sky'.
I have one shape, but I am two things at the same time.
He's nor angel, nor a human. Though he;s exists in same form being summoned on earth.
If you miss your body or your heart, be afraid of me.
maybe this means people who do not follow 'their heart' and do mean things, along with those who are not humans (armies of hell\heaven, demons, etc.)
*wiki quote:
War is not aligned to Heaven or Hell but instead serves to bring order by the rule of The Charred Council, whose purpose is to uphold the balance between the two forces.
I know this might not exactly fit, but it's first what came to my mind, and seems pretty logic :)
Is the game
Bayonetta
Because:
I fight in the battle between Darkness and Light.
The plot of the game as an Umbra Witch vs God's creations.
My owner is the sky.
Bayonetta's father was part of the Lumen Sages, basically opposites to the Umbra Witches. Since Heaven is usually seen as "up above" (the sky), you could say Bayonetta partly belongs to the Sky.
I have one shape, but I am two things at the same time.
She has the shape of a woman, yet her hair can change into beasts (Gomorrah) and weapons, like fists for example.
If you miss your body or your heart, be afraid of me
There are Broken Witch Hearts throughout the game, which increases Bayonetta's power if they are collected. So if you miss your heart they are found, and you should be afraid