The Royal Slave outfit consists of you wearing a silver circlet on your head and a figure hugging white basque on your body. The appearance of the headgear is triggered by involuntary lactation, with the chance increasing each time it occurs. This means the only way for the cow ears to appear is by using the milking bench.
The royal guard will never ever attack a royal slave until provoked (lest he suffer the wrath of the super-powerful non-super-slut real princess); but as usual once unfriendly he will return for repeat sessions.
The royal slave currently has no other major effects. But the basque does, and the only way you can get it is via the circlet. The basque does something rather unique with lactation which I'll leave you all to discover the details off. Put it this way, the job of the royal slave is to produce and store royal milk, and to keep it on tap.