One of the key traits of Atlus' Persona series involves building social links. This entails side-quests where you spend time with characters, build stats, and learn more about their story, and this includes both party members and NPCs alike.
In Persona 5 and Persona 5 Royal, it's possible to romance multiple characters. While this idea isn't heavily recommended, players can learn about the characters, though note that some become more useful in battle than others. Please be mindful of spoilers below. This will not include Persona 5 Royal's 3rd Semester storyline spoilers.
10 Ann: High Energy
Ann Takamaki wants to focus on her modeling career. After the incident with Kamoshida, she maintains her modeling while taking care of her close friend, Shiho. However, she encounters a rival model who attempts to upstage her during their social link.
While boosting Ann's confidant and confidence, the player will gain her Sexy Technique and Crocodile Tears. These will prove useful during negotiations in battle by making the enemy demand less. Furthermore, upon awakening Celestine, Ann will gain High Energy, which allows the party to deal double magic damage.
9 Haru: Gardening and Life Wall
Haru's social link involves taking over her deceased father's company, Okumura Foods. Along the way, she'll constantly be courted by her arranged fiance. But, perhaps the biggest secret is Haru's passion to open up her own cafe and follow her grandfather's footsteps.
In the meantime, akin to Persona 4, players can garden rare and effective healing items alongside her social link. Furthermore, awakening Lucy grants Haru her final ability, Life Wall, which grants a barrier to the party. She can block all attacks on all party members once.
8 Makoto: Checkmate
When exploring Makoto's social link, it becomes readily apparent that she's been sheltered and struggles to pick up on social norms. Along the way, however, she meets a new friend who she relates to. Unfortunately, the girl ends up becoming a victim of an abusive boyfriend, and Makoto wants to protect her.
While helping Makoto save her friend and learn the truth, she will open up to the player. Maxing out her social link eventually awakens hew final Persona, Agnes, along with the ability Checkmate. This new ability allows her to debuff the entire enemy party.
7 Futaba: Ultimate Support
After being sheltered and surviving a traumatized past, Futaba wants to live life as a normal girl. When she's not memeing in real life, her social link includes checking off her bucket list. She even plans to attend school one day.
Leveling up Futaba becomes crucial simply because she's the party member that stays no matter who else is in the party. As such, her ability to buff and heal the party at random strengthens with each social link increase. Upon awakening Al Azif, her Ultimate Support ability will allow her to strengthen and optimize these abilities when needed most against powerful boss fights.
6 Yoshizawa: Masquerade
The new character introduced in Persona 5 Royal, Yoshizawa, becomes a crucial character in the storyline. In order to progress into the third semester, it becomes mandatory to max rank her link. In doing so, she'll strive to become the best gymnast she can possibly be.
Yoshizawa's story is among the most tragic in the game. However, this Cinderella story's Persona user not only has a secret penchant for cooking but eventually learns to embrace her own talents. Once Ella awakens, her Masquerade ability will dole out Severe Physical damage to a single enemy twice.
5 Chihaya: Special Fate Reading
Chihaya's social link starts with her essentially ripping off the player for a stone, which she then sells for 100,000 yen. However, she constantly insists that her powers are legitimate. Eventually, her social link reveals that she is a true fortune-teller, but she binds herself by the chains of fate without changing her own destiny.
While not a party member, this young fortune teller does carry benefits. For those seeking to progress their social links, her readings will allow closer bonds with party members. This will allow players to save time by bonding with their allies faster.
4 Ohya: Legendary Scoop
At first, Ohya comes off as a carefree lush. In truth, she's driven to work overtime while being forced to avoid a story she wants to pursue. In doing so, she'll trace the whereabouts of her friend and former journalist partner.
Ohya's abilities do not help as much for stealthy players. Her ability, Legendary Scoop, which lowers security levels in palaces, becomes a minor help at best. However, at least Lala Escargot keeps her in check and gives the player a place to work.
3 Takemi: Resuscitation
Dr. Tae Takemi runs a private clinic and sells medicine in Yongen-Jaya. Unfortunately, she was ousted from her hospital job by a duplicitous higher-up. While selling medicine to the party, she'll also conduct experiments with medicine on the protagonist.
All this comes from the desire to save a little girl's life. The protagonist's ability to help her with medical trials builds trust and enables Takemi to sell more revival medicine. While it's possible to get HP and SP medicine from numerous sources, Takemi's medicine, at the highest level, becomes the best for revival, status ailment curing, and group healing.
2 Kawakami: Special Massage
Sadayo Kawakami works as a teacher during the day while moonlighting as a maid. Her job entails being a sex worker, at which point she even considers quitting her teaching job in order to be a sex worker. Her reasoning comes from paying off a massive debt over the guilt she has over a student that died.
While lifting Kawakami's burden, she will grant the protagonist extra time for activities in class, make coffee, and cook curry. All these also boost with Sojiro's social link ranks. Furthermore, finishing her social link allows the player to go out after days at the palace or Mementos.
1 Hifumi: Togo System
Hifumi Togo dreams of becoming a champion Shogi player. However, her mother stands in the way of her dreams and her desires. Upon befriending her, she will teach the player her Shogi tactics.
In addition to boosting Knowledge with each social link, her abilities become extremely prominent in changing the landscape in battle. By allowing party members to switch out, players can exploit weaknesses, heal, buff, and utilize new strategies. Furthermore, her confidant also allows players to gain extra money upon a successful ambush when ended in a single turn.