Roscoe adores Elizabeth. She is his playmate and provider of dinner. His very favorite thing is to wake her in the morning. They delight in playing together.
This morning Elizabeth was sitting in a box lined with a blanket, pretending to be a puppy. Bing and Roscoe at my feet, muzzle fighting. Kate at loose ends, kind of wandering around.
Then Kate got the idea that maybe she would like to lay in the box that Elizabeth was occupying. She nosed at Elizabeth and was asking Elizabeth to relinquish the box. Elizabeth protested.
Roscoe broke off from playing.
He interposed himself between Kate and his Elizabeth. 
And herded Kate away.
Roscoe nuzzled Elizabeth to reassure her, then returned to muzzle fighting with Bing. Kate walked off in apparent confusion (“Huh. Did that just happen?”) and got a drink of water.