It’s one of my favourite archetypes so I’m biased but may I suggest Courtly Hunter? The archetype trades out most of its combat abilities for an intelligent pet and social abilities; however you can get back some cool combat cantered focuses with the Planar Focus feat you can take at 5th level like fire damage added to all melee attacks for example. You can take boon companion and dip 4 levels into a combat focused class like fighter to get a few more combat abilities or feats without loosing too much from the hunter class.
There are really too many ways to go with this. Some classes are more "noble" by default, but almost anything could be built as a noble/royal.
You might try a swashtigator. That's where you take one level of inspired blade swashbuckler and then follow that up with levels of investigator.
Rogue Phantom Thief is the most social and skilled of the rogues, especially if unchained, and it natively has a noble association.
If leadership is allowed, you might try going for the noble scion prestige class.
What class or archetype you choose will have minimal impact on building a royal character. This is more about skill selection and maybe what feats you takes. Almost any class with the relevant class skills will work. Obviously a class with more skill points will allow you to put more skill points to the appropriate skills. But even a class with only 2 skill points can work. A human paladin with a 12 INT and putting his favored class bonus to skills gets 5 points per level. The feat Cunning from the Villain Codex would boost that to 6 per level, which matches anything but a rouge.
That being said my advice for a potential ruler would be bard. Bards are the jack of all trades which is prefect for a royal character. Bards have all the social skills as class skills bardic knowledge means they are knowledgeable on nearly everything. Since a ruler will be making decisions on everything in his kingdom this is incredibly useful. Versatile Performance is also going to be useful especially anything that replaces Sense Motive. Being able to use your casting stat (probably your highest stat) for sense motive is very good.
Bards are a medium BAB class with a good selection of weapons, and the ability to cast spells in light armor. This gives them enough combat ability that they are not helpless and can survive for a while when they are confronted with combat. A few combat feats can give them enough ability that they can hold their own.
Bardic Performances are also something a royal would find useful. A king using inspire courage on his army could easily turn the tide of the battle. Bardic performance are usually limited to those being able to perceive the performance. Clarion Call is a first level bard spell and allows the caster to be heard over great distances. With this a bard can use inspire courage on huge numbers.
The bard spell list is also well suited for a ruler. Not only do they have a lot of buff spells, they also have a decent number of divination, enchantment and illusion spells. Being able to use spells like detect magic, ears of the city, detect thoughts, and scurrying allow the ruler to gather information without having to rely on others. Spells like disguise self, enshroud thoughts, misdirection and false future allow the ruler to prevent his enemy form gaining information on the ruler. Heroism gives the character a long lasting buff on just about everything he does. Dispel magic allows him to remove hostile magic or to protect himself (or his subjects) from hostile magic.
Bards are also a CHA based caster with UMD as a class skill and plenty of skill points. This gives them the ability to potentially use any magic item in the game.
Not to toot Courtly Hunters horn but I do want to point out that with your intelligent pet (equal to a familiar) the class also grants it all your class skills and skill ranks like a familiar (as well as its own ranks) you therefore essentially get potentially 2 rolls on every skill check or dedicated aid other on one roll. The teamwork feat Bonded Mind lets you communicate telepathically with your pet so u can communicate easily. Enuff on the Courtly Hunter; the Vigilante class could also do well in the roll of Royal Hero.
Don't forget the most "social-focused" martial: vigilante. Some archetypes are even primarily focused around a "court"/"spy" campaign (and can be adapted for other nations fairly easily): Dragonscale loyalist, imperial agent, teisatsu.
Another option to consider is Variant Multiclassing. VMC Bard can make any class a budget Bard at the cost of feats. VMC barbarian can add combat potential to even the tamest of classes/archetypes. VMC rogue or VMC cavalier with order of the blossom can give you up to 4d6 sneak attack (5d6 with accomplished sneak attacker feat). VMC witch/wizard/sorcer(arcane bloodline) can give u a familiar and the Mauler familiar archetype can turn it into a budget Animal Companion. VMC monk on an Elemental Ascetic Kineticist is a potent combination giving the class scaling monk damage.
The real question is what type of character do you want t play? Saying you want to be royal is really no help. Are you looking for something that would be an effective ruler, or something that is more heir to the throne? Do you want more of a martial character or more casters? My recommendation of bard was because they make incredibly good rulers
Another option would be paladin. This would work really well for an heir to the throne character that is a champion of the people. If the current king is less than scrupulous but not outright evil, this could be a form of rebellion. Your character could stand up to his father when his father is being less than noble. A good example of this would be Arthur Your paladin's powers could be more from divine right than training. If you want a pure martial a cavalier would be another option. This character would be similar to the paladin, but without the religious and alignment restrictions.
What is the culture of the campaign? If it is a more medieval culture than renaissance the swashbuckler types may not fit as well. If the campaign does have a culture similar to the renaissance those may fit.
If the swashbuckler types fit the dandy ranger would be very good. It has plenty of skill points and the right class skills. The bard spell list as I mentioned is quite good for a royal so that is another plus for the archetype. If your GM allows it the Cayden Caliean style would work really well. Use your normal feats to get weapon finesse, weapon focus and fencing grace and the style feats to pick up combat expertise, improved disarm and disarming strike without needing the 13 INT.
You could do noble scion it seems like it would work well. Its a prestige class from path of prestige
There are several good prestige classes in Pathfinder. It’s just that there are also more than a few bad ones.
If the idea of an intelligent animal companion interests u but u don’t want to take the Courtly Hunter there is another way to get one. Depending on ur alignment you can get either a griffon (INT 5) or a warg (INT 6) with the monstrous mount feat. Both Human and Halfling have alternate racial traits that can add +2 to any stat u choose for Animal Companions or familiars. The Precious Companion pet archetype adds 2 to INT and 4 to WIS and CHA. With these options and the stat bonuses they get every 4HD you could have a griffon with INT 8 at level 5 going up to 10 at level 7. U can do this on any class with an animal companion so it opens up ur options a lot. If you build for it you could let ur animal companion take care of your intimidation rolls while you handle the polite social interactions.
I would strongly discourage the Noble Scion. Prestige classes are usually a trap in Pathfinder especially for spell casting classes. Noble Scion is worse than most in that respect. All it really gives you is some wealth related benefits and improved followers. If you are a sell caster you are sacrificing 8 caster levels which is way too high. If you are a martial you the medium BAB is really going to hamper your combat ability.
From what you have said it sounds like what you are looking for is a CHA based character with some magic ability and decent skills.
I am not familiar enough with the mesmerist to give you any useful advice.
The Courtly Hunter at first seems like it might work, but it is not a CHA based class. Its spells are WIS based and is also fairly combat oriented. That means you will probably want decent physical stats and will need a good WIS to cast your spells. Most of its spells are nature themed and do not lend themselves to your concept. If you are ok with reducing the characters leadership qualities this could work, but if you really want to play a leader this may prove difficult.
There are some spell casting vigilante archetypes. The magical child is the only CHA based caster for vigilante, but they use the summoners spell list, which does not have much in the way of enchantment. They also only get 4 skill points per level. The warlock is an INT based caster that uses the wizard’s spell list. This would give you access to enchantment spells for control. They also only get 4 skill points per level but as an being an INT based caster; they will actually have more than that. Unlike the Hunter they also get mystic bolts so don’t need to use weapons for combat. This means they don’t need to worry about STR as much so can still get some CHA.
I would strongly discourage the Noble Scion. Prestige classes are usually a trap in Pathfinder especially for spell casting classes. Noble Scion is worse than most in that respect. All it really gives you is some wealth related benefits and improved followers. If you are a sell caster you are sacrificing 8 caster levels which is way too high. If you are a martial you the medium BAB is really going to hamper your combat ability.
From what you have said it sounds like what you are looking for is a CHA based character with some magic ability and decent skills.
I am not familiar enough with the mesmerist to give you any useful advice.
The Courtly Hunter at first seems like it might work, but it is not a CHA based class. Its spells are WIS based and is also fairly combat oriented. That means you will probably want decent physical stats and will need a good WIS to cast your spells. Most of its spells are nature themed and do not lend themselves to your concept. If you are ok with reducing the characters leadership qualities this could work, but if you really want to play a leader this may prove difficult.
There are some spell casting vigilante archetypes. The magical child is the only CHA based caster for vigilante, but they use the summoners
spell list, which does not have much in the way of enchantment. They also only get 4 skill points per level. The warlock is an INT based caster that uses the wizard’s spell list. This would give you access to enchantment spells for control. They also only get 4 skill points per level but as an being an INT based caster; they will actually have more than that. Unlike the Hunter they also get mystic bolts so don’t need to use weapons for combat. This means they don’t need to worry about STR as much so can still get some CHA.
I agree with you noble scions are far from great but there ablities fit well with a noble character.
I think the first question is what kind of realm does your royal character come from? Its culture, laws, etc., should provide ideas for what type of character he could be.
A more martial realm might expect its royals to be cavaliers, for example. And within that example, a more dashing prince might be a Daring Champion, while a more cerebral one might be a Daring General. A magocracy, on the other hand, might require its royals to study as wizards. Alternately, a prince seeking to reform a corrupt realm from within may become a vigilante--with his alternate identity being that of a rebel to the crown!
If you want to take on the role of the rouge the Archaeologist Bard would be perfect. They trade out bardic performances for rouge abilities. Ignore the bard label and build the character like you would a charismatic rouge. For Traits you will want fates favored and criminal. The archetype does not give disable device as a class skill, but criminal will fix that. Fates favored boost your archaeologist’s luck by +1. Luck bonuses are incredibly rare so Archaeologist’s Luck usually stacks with everything. You will want the following feats weapon finesse, lingering performance and a feat to get you DEX to damage. Make sure to take heroism as one of your second level spells ASAP. By 5th level the combination of Archaeologist’s Luck and Heroism will give you a +5 to nearly everything.
Instead of the heir to the throne you are second son. You preferred the company of rogues and whores because you thought they were more honest than the nobles. Your luck bonus is actually due to a vestige of divine right. The gods have plans for your character, but your character has no clue about that. You just want to live your life as you see fit, so left home to get away from it all. A time will come when your character saves the kingdom from doom, but this is not something you are aware of yet.
I have built similar characters without the royal aspect, and they have always been fun to play. If you focus most of your spells on boosting your “thief” abilities, you don’t need a sky-high CHA. The bard spell list has a lot of spells that work for this. If you think a normal rouge is good try one that can change his appearance with illusion, turn invisible, assume a gaseous form and teleport short distances.
Even though you are technically a bard you don’t have to put a single rank into any performance skills.
There is also no reason to take any levels in aristocrat.
Noble Scion works better with non-caster builds. Swashbuckler is an ok entry point, but any charisma friendly martial should be good for this.
You may consider building around Desna's Shooting Star so you can get charisma bonus to attack and damage.