Invisible friend

I had a twin that couldn't be seen.
Maybe he wasn't real and
has never even been?
But perhaps he hasn't left me...
Sometimes I sense that he is near,
and his distant laughter I can hear.
Occasionally glimpses of him I see,
or is the visage in the mirror only me?
Perhaps he just couldn't stay?
As I grew older he had to go away.
I shouldn't be sorry and will not grieve
for this did he me promise:
"I will never leave."