The long-running Persona series is known for its fusion of RPG gameplay and simulated school life, with the formula being firmly established in Persona 3. The games' protagonists are historically mostly-silent and leave a lot to the players' imagination. However, when they're compared to each other, their individual characters become more apparent.
The mainline Persona games have seven protagonists across the series, and they do have traits in common; all of them are natural leaders and have personalities that draw other people towards them. However, the way they interact with other characters and how they view themselves change in each game.
Persona 1
Out of all of the protagonists, Persona's main character is perhaps the most "blank slate." While the manga names him Naoya Toudou, the only official name he receives in the actual games is "Boy with Earring." He does not lean particularly towards any sort of personality as a mostly-silent protagonists aside from the obligatory trait of "natural leader."
Side content and flavor text fill in some of the gaps for Naoya; for example, a comment Mark makes suggests that Naoya isn't particularly studious. He apparently pierced his own ear at the age of 12, suggesting some extent of impulsivity or rebellion. While Naoya's personality is open-ended in his own game, Yukino in Persona 2: Innocent Sin describes him as a friend who would gamble even when they had serious problems, which suggests further that Naoya is an impulsive person. While the protagonists from Persona 3 onwards are all associated with The Fool, Naoya's associated arcana is The Emperor.
Naoya is not as fully explored as the other Persona protagonists, and Atlus likely won't bring the original game back anytime soon. Either way, Naoya has served well as the base foundation for the others to be built on.
Persona 2
As there are two Persona 2 games, there are two protagonists. Tatsuya Suou is the protagonist of Persona 2: Innocent Sin. While some aspects of his personality can be influenced by the player, Tatsuya is one of the most fleshed-out protagonists in the franchise. He's seen as a "bad boy" archetype by his peers, and is a brooding loner. No matter what choices the player makes, his closest bond will be with Jun.
He appears to be a bit lost as to what he wants to become in life, and spends most of his time working on his motorcycle. Tatsuya is currently the only bisexual (or otherwise LGBT) protagonist in Persona. Tatsuya can express his feelings for Jun, and from thereon the two are seen as romantic partners. Tatsuya's associated arcana is The Sun.
Maya Amano is the protagonist of Persona 2: Eternal Punishment, and she is the oldest protagonist of a Persona game at 23 years old. Maya's personality is also well-established; she's known for being a positive and kind person. Compared to Tatsuya, she is much more extroverted and optimistic. Her associated arcana is The Moon.
Persona 3
Persona 3 started the gameplay formula that other games in the series would follow. Makoto Yuki, also known as Minato Arisato, is the first protagonist associated with the Fool arcana. Makoto is an orphan who appears to have a slightly gloomy personality, as he is quiet, stoic, and can potentially make morbid jokes or comments depending on the player's choices. Makoto's reserved and blunt personality causes some of his peers to find him standoffish, but he clearly still cares about his friends.
Persona 3 Portable's female protagonist has no official name in the games, but the stageplay names her Kotone Shiomi. She is radically different from Makoto in both design and personality. She's funny and cheerful, even though she too is an orphan with a tragic backstory. She is outgoing and dependable, and just a bit more hot-headed than Makoto. While the female protagonist is not technically canon to the Persona series' story, she is very popular and returned in Persona Q2 along with her male counterpart.
Persona 4
Persona 4's protagonist is officially named Yu Narukami, though he also has the name Souji Seta in the manga. Yu's personality varies a bit across the many Persona 4 adaptations and spin-offs, but he is most consistently depicted as a very quiet but friendly character. Yu is cool-headed and mature, and his sense of humor is dry and subtle.
While social bonds are important to all of the protagonists, Yu's friendships are particularly integral to his character. The Investigation Team is arguably one of the most tight-knit parties, if not the most. The time the party spends together as high-school friends feel more natural than the Phantom Thieves for instance. Yosuke and Yu are connected in a special way as "partners," and that bond is steadfast throughout all Persona 4 content. Yu's dedication to his "little sister" (actually his cousin) Nanako is loving to an extent it's exaggerated to comedy in the Persona 4 Arena games. His biggest insecurity is in fact his need for other people; without his friends, he feels weak.
Persona 5
The manga names him Akira Kurusu, but the Persona 5 protagonist's most official name is Ren Amamiya. Ren takes after the first two games' protagonists in a sense, with his bad boy image at school and rebellious spirit. His dialogue options suggest a snarky personality, and he often gets the opportunity to tease other people, including his friends. His intensity belies a compassionate center; he has a strong sense of justice and does not want to see people being taken advantage of.
Ren is a bit of a showoff, which Futaba comments on during Persona 5's opening animated cutscene. He seems cockier and more flamboyant in his Joker persona, often adding flourishes to his attacks. His main persona, Arsene, has a very grand and imposing personality, reflecting Ren's inner self.
The Persona series is carried by its characters, and it will be exciting to see what kind of characters Persona 6 will bring into the series. Hopefully, its protagonist will have plenty of personality and just as much of a compelling character as its predecessors. 2021 is expected to be a big year for Persona, so announcements may be on the horizon.
Persona 5 is available now on PS3 and PS4, and Persona 6 is rumored to be in development.