Castle, symbolically. Yes he’s not her boyfriend, but Rhodes clearly sees the unresolved sexual tension between, uses it to try and seduce Castle, takes coffee from Castle (symbolically very important), asks Beckett for permission to sleep with Castle, Beckett gets jealous over Castle being attracted to Rhodes etc.
Rhodes never even meets Josh, or even is aware of his existence.
While that ep is hilarious for all sorts of reasons, it is extremely important for underlining just how possessive Beckett is of Castle and his affections in the aftermath of his break-up with Gina (remember that the filming order was different to the airing order for NH and PYD).
IM IN A MESS OF EMOTIONS BUT THANK YOU FOR PUTTING UP THE SCENES
emma & killian + 3.17 the jolly roger
'How am I supposed to trust a man who no longer believes in love?' ‘I still do.’