Ninja

You can obviously see me now,
But only for a little while,
Because if you will look again,
I will be gone without a smile;
I am here, or I can be there,
I'm in the shadows or I'm not,
You can try to look everywhere,
You're always cold, you're never hot;
So try to catch me if you can,
I'll appear when you least expect,
In front or in the back of you,
I am Ninja, don't you forget.