You know my love, be good

You must not cry

You know why

Santa Claus is comin' to town

He notes everything, he sees everything

And he follows your steps wherever you are

Santa Claus is comin' to town

He watches you when you sleep

He watches you when you wake up

Don't try to hide from him

Because he will always see you

He knows about you

He knows about me

He knows everything

Don't try to run away

Santa Claus is comin' to town

Grumpy Santa Claus, there is no extensive backstory, he just goes along with the plot at his own pace.