I have a level 3 ninja in PFS and I'm considering a dip into monk levels so I can add the martial arts aspect to him.
Is it worth it to do this, or is there another / better way to accomplish this goal? (I would of course use the unchained monk.)
Depends on what it is you exactly want out of your dip.
Improved Unarmed Strike? Damage Dice? AC bonuses? Bonus feats?
Should be workable, assuming you're not relying on your armor too much for defence and don't mind the delayed ninja trick and sneak attack progression (though you could always take Accomplished Sneak Attacker as a later feat on the justification that you're getting a bonus feat from being a monk).
The flurry from Unchained Monk is always nice if you use unarmed attacks or monk weapons (I'd suggested using a monk weapon given you're only dipping and won't progress the UAS damage die
-- the new waveblade is nice), and the ki pools will stack, subject to the rule about applying only one ability score modifier.
The Black Asp archetype is most flavourful for combining with a ninja, but if your current character is more CHA-based than WIS-based, you will probably want to look at Scaled Fist. Both replace Still Mind, so you could only choose one, ad the Black Asp doesn't really grant anything for a one-level dip, so Scaled Fist seems like the right choice.
What are your ability scores right now? Having a good CHA score will definitely soften the blow of losing your armor (assuming Scaled Fist).
I would also consider looking into the Master of Many Styles OG monk archetype, it'd get you the ability to use two combat styles at the same time as well as a free style feat. Although you do lose out on Flurry, but since you're only taking the one monk level anyway thats not too much of a loss.
Thanks for all the advice so far!
I realized after reading the various monk styles and archetypes that my ninja's alignment isn't lawful, so he would need to undergo an alignment change before he could dip into monk. And I'm not sure how that's accomplished. I don't see anything regarding it in the PFS rules or in the retraining rules ... so does my character just change alignments between scenarios? Or am I missing something?
I'm liking the Brawler option. No alignment change needed, and my intention for the martial arts aspect is more for flavor than for mechanical advantage. I just have a hard time envisioning a ninja that doesn't use martial arts in some way.
Mechanically Ninja and unchained monk synrgize quite well. Especially scaled fist monk as it uses Charisma.
Not sure if alignment changes in PFS are kosher.
You could consier Kharmic monk as it doesn't require you to be lawful.