Ants are unisex though some of them are clearly more female than male. Pitou is probably supposed to be male despite being described as a 'mother'...
Assuming you get the Royal's unquestioned devotion (otherwise Pufu is definitely out, too risky), the next question is what do you intend your bodyguard to do. If it's anything less than 'taking on the Hunter's Association' then Pitou is a sure bet because they're all each more than capable of taking out a small nation (Hunter's Association is clearly stronger than a small nation) and Pitou wins for just having the possibilty of being female for any male, not to mention Pitou's special ability is extremely useful as it's pretty much one of those 'can recover from anything as long as you didn't immediately died' and it's pretty hard to beat that to play it safe.
But let's say you're at war with the Hunter's Association so you're actually fighting stuff that will test the limit of your new bodyguard. You'd have to consider a few things. The physical fighting power of someone is directly related to how far away from Reinforcement they are, so we get Yupi > Pufu > Pitou. From looking at stuff like the extent of En we have to assume in terms of aura mastery it's Pitou >>> other 2 (En does not seem to tie to any school and Pitou is way better at it than anyone else). So Pitou would seem like the no brainer since he even seems to be able to fight Uber Gon with some limited success, so although he's the weakest physically it's probably enough because no human is even close to Uber Gon's strength.
But there's a big problem with picking Pitou as your bodyguard. He has no attacks to deal with multiple strong attackers. The puppets are clearly useless on a strong opponent, so he'd have to dispatch the enemy 1 by 1. Although he seems to be by far the fastest of the Royals, it's still a disadvantage compared to Yupi who can just go giant form and block off a chokepoint while fighting multiple guys, or Pufu who can flat out fight multiple guys at once. You'd have to have some knowledge of the nature of the enemy you're fighting. For example if the enemy had 2 Morels and assuming you're a totally normal guy, you'd be pretty screwed because one Morel can definitely keep your Pitou bogged down while the other one kills you, while the other two Royals have abilities that can fight 2 Morel level guys at the same time (not necessarily effectively, but better than nothing).
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