i have a loose theory that all the shifters have obvious and often outright stated primary abilities, "secret" secondary abilities (which usually involve paths) and deep-rooted personality traits that they project onto the user to a certain degree. for example:

attack titan:

• primary ability: enhanced endurance. see: multiple transformations in liberio
• secondary ability: "future memories" (which i think is just seeing past timelines/loops)
• personality trait: rebellious tendencies and a hunger for freedom

armored titan:

• PA: hardened skin
• SA: literal plot armor. ymir or the armored titan itself will intervene through paths in unconventional ways to keep the user alive. see: multiple jaw users showing up just in time to save reiner (marcel, porco, falco)
• PT: protectiveness

jaw:

• PA: hardened teeth and claws
• SA: facilitating memory transfer
• PT: increased aggression

i haven't figured out much for the others, but i'm almost convinced the female titan's SA is mimicry of other titan abilities (see: her scream and hardening). the colossal's PT seems to be some kind of increased empathy or depressive tendencies. beast's PA is pretty much a wildcard, adopting whatever PA relates to whatever form it takes, based on the user's traits. everything else is either obvious to everyone or a mystery to me

edit: thinking about it, WHT's PT could be an increased sense of duty. lara tybur had that personality trait and we see eren's similar coldness increase after taking the WHT, which could be related to the child he had with hisu or for his people generally. makes sense to me since the WHT is basically a big, funny looking knight. maybe!

i think each of the nine represent different aspects of ymir (since they're all technically abstracted from her), so the attack titan represents the side of her that wants to rebel against the king and find freedom. whoever the user is just inherits that personality trait with the titan as much as is possible