I went to Desdemona and she told me to go on an "op with Deacon" and now she won't say anything else. Who the hell is Deacon? There's no-one of that name in the Railroad HQ and there's nothing on the map.

Hes a companion
And the railroads best scout/operative

He has black hair, wears sunglasses and a white top.

The first mission you do with him is an operation to save some synths - but only if you want to join the institute

He vouches for you when you meet the railroad for the first time. Im on my 3rd playthrough and each time ive seen him so.. Not sure how youve missed him.

I've not met anyone called Deacon yet. Where is he? I can't complete the main quest because of this.

For me, Deacon was the fast talker who appeared when I met Des and the other once I had completed "Walking the Railroad" quest.

Fast talking on my side, no less, even though I was pretty far along in the BoS at the time.