Sources have finally confirmed the identity of the mystery star set to hit Castle in the May 7th finale, "Always". Judith Scott will reprise her role as Captain Montgomery's wife, Evelyn. Creator Andrew Marlowe spoiled some of the deets here.
“Events force us to wonder whether the case we’re working on might be linked to the Beckett’s mother murder case” – aka the conspiracy-led cover-up that Monty was rather complicit in, and ultimately died for. “We’ll be picking at that scab again.”