The World of Eternal Sonata features a number of characters, both playable and unplayable.
- Frédéric François Chopin (フレデリック・フランソワ・ショパン Furederikku Furansowa Shopan?): Main protoganist and antogonist, 39 years of age. He is a renowned composer and pianist. Fighting with batons, he travels through a world he believes to be entirely his dream. As he does, his certainty that the world around his only a dream begins to waver and he searches for a way to save the young girl he has met, Polka.
- Polka (ポルカ Poruka?): 14 years of age. Polka has powerful magic abilities, which in turn means that she is doomed to die. Fighting with a parasol, she travels in the hope of using her abilities for good, knowing she doesn't have much time to live. Her encounters with Frederic help her to have a more optimistic outlook, and also drive her to face her fate.
- Allegretto (アレグレット Areguretto?): 16 years of age. A young man standing up to the contradictions of the world. Though Allegretto is poor, he has a good heart. He is a thief who steals bread in order to feed children who are not able to feed themselves. Fighting with swords, he has taken his companion, Beat, under his wing and has quickly developed an affection for Polka, the young girl he meets in Agogo Village.
- Beat (ビート Bīto?): 8 years of age. Beat is a young boy who lives with Allegretto in the port city of Ritardando. His greatest treasure is a camera which was given to him by his father. He utilizes a hammer that can transform into a gun and generally maintains a cheery outlook on life despite the hardships he has faced. He often finds himself at loggerheads with Salsa, one of the Guardians of Agogo Forest, but later forms something of a friendship with her and the beginnings of a young crush.
- Viola (ビオラ Biora?): A 26-year-old shepherd the party meets in the countryside. She's a tough-talker and can handle herself, being slightly older than the others in the party. She has a pet named Arco who tags along with the group. Fighting with a bow that can also be used to whack opponents at close-range, she regrets not having acted sooner when she knew something was wrong with the world, and has developed a crush on Jazz, the leader of the Forte resistance group Andantino.
- Salsa (サルサ Sarusa?): An 8-year-old guardian of the Agogo Forest with her sister. The party meets her when they were imprisoned in the Forte Castle dungeons. Salsa views hats as the best treasure in the world. Salsa has a tendency to speak her mind and is not afraid to rush into a situation. Fighting with two large rings, her lack of refined manners immediately rubs Viola the wrong way and leads to a childish rivalry with Beat, though the two later largely resolve their differences.
- March (マーチ Māchi?): The other guardian of the Agogo Forest; Salsa's twin sister. She tends to be the more reasonable one of the two, making her a sharp contrast to Salsa. March appears to be mature and gentle. March speaks with a wisdom seemingly beyond her age and is a scholar of ancient texts. Fighting with rings similar to her sisters', she is somewhat less used to battle than Salsa, but nevertheless proves to be a formidable fighter.
- Jazz (ジルバ Jiruba?, his Japanese name is a transliteration of "jitterbug"): Jazz is 27 years old, leader of the revolutionary group Andantino. Quiet and serious, he worries about the damage Count Waltz might be doing to the people with the mineral powder and the processes needed to mine it. Jazz was originally involved in a revolution of the Forte miners which ended in defeat. Fighting with a massive broadsword, he is able to deal massive damage to large groups of enemies.
- Falsetto (ファルセット Farusetto?): Jazz's lieutenant in Andantino, she's perceptive and tough, and inwardly dislikes Claves. Falsetto is 22 years old and has known Jazz since childhood. Falsetto may sometimes run away from a problem she finds too tough to face, but will return in the end to fight for what she believes to be right. She fights with knuckles and gloves and proves to a be swift and agile fighter.
- Claves (クラベス Kurabesu?): Jazz's 24-year-old girlfriend and another soldier of Andantino. She is a later addition to the group and quickly moved up the ranks due to her skills in battle. Her tendency to need details of missions clarified leads Falsetto to quickly become suspicious of her. She fights with a rapier and begins with low stats, but later gains powerful stat growths.
- Crescendo (クレッシェンド Kuresshendo?): The young prince of Baroque. Crescendo replaces his father for leading Baroque into the future war between Forte due to a sickness his father is suffering on. He rescues Polka, Beat, Frederic, and Salsa after they fell into Fusion River. Crescendo is worried about the strain that Baroque's contentious relationship with Forte has placed on the country and is searching for a peaceful solution. Only playable in the PlayStation 3 version of Eternal Sonata, Crescendo fights with maces and specializes in high HP and strong defense.
- Serenade (セレナーデ Serenāde?): The fiancee of Prince Crescendo. Originally from Forte, she too searches for peace. She owns a pet dog named Minuet. Like her fiancee, she is only playable in the PlayStation 3 version. Fighting with Staves, she is incredibly fast in battle and carries some of the game's most heavily damaging Special Attacks as well as a powerful healing skill.
- Count Waltz - The young leader of Forte, he has developed the dangerous mineral powder.
- Legato - Waltz's lieutenant; he obeys his every command.
- Tuba - A large brute of a man who leads Forte's military.
- Fugue - A monocled agent employed by Waltz to search for glowing agogos
- Rondo - An assassin employed under Waltz