Harry seems as interested in Darby as Darby is in him.
Darby charges after him and pounces on him.
Licks his ears until he jumps up and makes a half-hearted attempt to get away.
After letting her lick his tail, he decides he's had enough.
But, notice he doesn't run too far.
He doesn't run up the stairs where she couldn't get him!
I think he wants to play as much as she does.