Asner and mister?

Because at the end Willie goes to live with Mr Tom Oakley forever and he can say, every night, Goodnight Mr Tom. The name of the novel comes from something William said early on in the story. Up to...

Mister Monday's key is the key that will let him continue his reign.

Domnu is a Romanian equivalent of 'Mister'.

If this is a crossword clue, your answer is 'señor'.

Mr