We have character concepts for mechanical things but usually the best characters are those that have a simple, contundent backstory that drives them.
From Hailey of OotS to save those she loves (and being misinterpreted throughout it), Jean Vanjean (Les Miserables, the pursuit of an honest life while keeping his identity secret), Edmond Dantès in an intricate revenge plot (Count of Montecristo), Dread Pirate Roberts (one of a long line, not really who you'd think he is, Princess Bride), Cassandra can see the future but is cursed to ever be thought a fool (Illiad), Constantine faces a dark end and does what he can to elongate his meeting with either god (Hellblazer)...all these have that spark that makes you buy into them.
Rather than the whole story revolve around them though it can be run as a backstory the GM can call upon.
The thief who took the wrong Coin
"Should even the most insignificant of
treasures depart Erebus without his leave, Mammon knows
as surely as one who has a digit sliced from his hand. The
archdevil’s schemes of reprisal and repossession come
swi! and terrible upon the heads of thieves, demanding
recompense in near-impossible proportions to what was
stolen." - Council of Thieves part 6
A thief stole what he thought was just another gold piece to later find that in fact it was part of the Archedevil Mammon's own hoard. She now travels Golarion with a very specific goal in purpose; the Prince of Erebus has named his prize. It could be something the party is also after. Can be twisted further with someone finding out that the coin was placed there on purpose and that unvalidates the contract, freeing the character.
Got any interesting ones? I guess these can work as NPCs as well.
The guy who, when Cayden Cailen said, "Hold my beer", did so.