The main protagonist.
- Adoption Angst: Anna starts out introvert and pessimistic about being adopted, especially upon finding out that her adoptive parents are getting paid for fostering her.
- Alcohol Hic: Happens to her after she gets a cup of wine.
- Ambiguous Disorder: Anna is frequently noted to act oddly for a child or focus on strange things. The book starts with her acting incredibly detached from other people, enjoying spending time alone, limiting interactions with the family she lives with, and sending postcards to her guardian that are only a few sentences long. She views the world as being a circle with people on the "inside" and on the "outside", believing herself to be on the outside and separate from everyone else. At one point, she goes into a fury over an insult which everyone, even the narrator, considers to be silly. She also doesn't seem to have many friends, but the people she does meet and bond with (Marnie and, later, the family that moves into her house) she's able to form very close ties to. Some of her behavior can be attributed to her fearing that her foster mother is only keeping her for the money, but her reaction to that still comes across as unusual for an upset child.
- Boyish Short Hair: Her hair reaches her jawline.
- Can't Hold Her Liquor: The single glass of wine as mentioned above is implied to be enough to get her snockered to the point of passing out afterwards.
- Character Development: Her adventures with Marnie transformed Anna from a sullen, angry person to something more along the lines of a Plucky Girl who begins to open up more.
- Color Motifs: Anna is almost always seen wearing the color blue — representing her earlier depression and later her being at peace with her problems.
- Dark and Troubled Past: Downplayed. Anna mentions her parents and grandmother dying when she was very young, leading to her being put into foster care, but she was only an infant when her family died, thus she hardly suffered any emotional trauma. And at age 5, she was Happily Adopted.
- Emotionless Girl: In the beginning, Yoriko comments that Anna used to express herself but is now attempting to suppress her emotions.
- Happily Adopted: At the start of the film, Anna is too pessimistic to see that her adoptive parents love her. A big part of Anna's inner growth is realizing that she has all she needs to be this trope, and accepts Yoriko as her mother by the epilogue.
- Heroic Self-Deprecation: Calls herself ugly and a parasite.
- Ill Girl: Anna has severe asthma and was sent to rural Kushiro for the summer because the seacoast has cleaner air, unfortunately playing right into her abandonment issues.
- Innocent Blue Eyes: Subverted. Anna's blue eyes serve as an ever-present reminder of her status as a (self designated) Red-Headed Stepchild.
- Intergenerational Friendship: She and Sayaka (both school age children) form a bond with Toichi (a man in his 50s or 60s).
- Meaningful Appearance: As Anna grows kinder and more open with others, she starts wearing a blue hair clip.
- Mixed Ancestry: Anna has blue eyes, while all Japanese have brown eyes. Anna turns out to be the granddaughter of Marnie, making her part foreign.
- Nice Girl: While she wasn't mean in the beginning, Anna could be when pushed. Her Character Development has her developing a better personality.
- No Social Skills: Not really good with socializing with people around her age.
- Raised by Grandparents: Anna was raised by her grandmother after the death of her parents. Until her grandmother died. Her grandmother turns out to be Marnie.
- Sailor Fuku: Anna's never shown in one herself but classmates of her's come over wearing them, showing that her school uses sailor fuku.
- The Stoic: Anna has become quiet and unemotional recently, which her mother refers to as her having an "ordinary face".
- Tomboy and Girly Girl: She's the tomboy and Marnie is the girly girl.
- Took a Level in Cheerfulness: Became more of a Plucky Girl by the ending.
- Took a Level in Kindness: Besides becoming more cheerful, Anna also became a nicer individual. One example is her referring to Yoriko as her mother and not adoptive mother.
- Tragic Keepsake: She gets two from her grandmother, Marnie: the blue hairclip and an old picture of Marnie's childhood mansion home.
The mysterious and titular girl Anna befriends.
- Break the Cutie: Behind her happy demeanor, Marnie has been put through a lot.
- Childhood Friends:
- With Kazuhiko.
- It turns out she and Hisako were also this.
- Childhood Friend Romance: Hisako reveal that Marnie married Kazuhiko, her childhood friend.
- Dark and Troubled Past: Where to begin? Her parents were always away and didn't attend to her needs. The governess and maids would abuse her once the parents were gone. The only man Marnie ever loved left her at the silo she was terrified of thanks to the maids for a night then later, after they were married and had a child, died, and due to the shock of it, she was sent to a sanatorium and sent Emily, her only child, to boarding school for her own good. However, when Emily came back, their relationship was strained and never had a chance to reconcile, even before Emily's death. Marnie was left with her only granddaughter, Anna, and tried to take care of her. But, Marnie herself died not long afterwards.
- Dead All Along: Since Marnie is Anna's grandmother, she's actually been dead for quite some time before the film starts.
- Doting Grandparent: Marnie promised her only grandchild, Anna, that she would never leave her alone.
- Hair of Gold, Heart of Gold: Marnie has beautiful blonde hair and blue eyes, and is quite nice.
- Healthy in Heaven: Exaggerated. Marnie is a perfectly healthy child despite dying as an old woman.
- Innocent Blue Eyes: Marnie has bright, blue eyes that represent her friendly and bubbly nature.
- Lonely Rich Kid: Her parents are hardly around and she never had a lot of friends.
- Missing Mom: Deconstructed. After Kazuhiko died, Marnie fell apart and she was sent to a sanatorium to get better. Their daughter, Emily, was sent to live at a boarding school. However, when the mother and daughter reunite years later, it's not a happy union. Emily felt hurt and betrayed at her mother's missing status for most of her childhood.
- Mixed Ancestry: Possibly. It's unclear if both her parents are white or if one is Japanese.
- My Greatest Failure: Not spending enough time with her daughter, Emily.
- Nice Girl: Marnie is polite, friendly, and even-tempered.
- Outdated Name: "Marnie" hasn't been a common name since the mid 20th century which is fitting, since she's not actually a modern child.
- Outliving One's Offspring: Her only daughter, Emily, died in an accident.
- Secondary Character Title: Her name is part of the movie's title, but Anna is the real protagonist.
- Single Woman Seeks Good Man: Marnie fell in love with Kazuhiko because he always supported her, especially when her family was torn apart or when she felt lost.
- Stepford Smiler: According to Hisako, while Marnie always appeared glad to show off the expensive clothes and toys provided by her parents' wealth, it was all a front for how lonely she truly was.
- Strong Family Resemblance: Marnie had her father's blonde hair and her mother's facial features.
- Tomboy and Girly Girl: She's the girly girl and Anna is the tomboy.
- True Blue Femininity: She mainly dresses in blue.
Anna's adoptive mother.
- Fostering for Profit: After discovering child support cheques, Anna is afraid that this is the reason Yoriko adopted her. Fortunately, it is not so.
- Good Parents: Yoriko thinks of Anna as nothing less than her own child. The reason she sent Anna away was to treat her asthma, and she frequently calls the Oiwas to inquire about Anna's well-being.
- Mama Bear: A passive example, but she takes Anna's health and happiness very seriously. The former is why she even sent her daughter to the countryside.
- My Beloved Smother: Downplayed. Yoriko isn't overbearing, only a concerned mother about her daughter's asthma problems and recent change in behavior.
- Nice Girl: She doesn't care about the money she and her husband get for Anna. She only wants what's in her daughter's best interest.
Anna's adoptive father and Yoriko's husband.
- Disappeared Dad: Downplayed. Mr. Sasaki is a part of Anna's life, but doesn't make an appearance in the movie because of his work.
- The Ghost: He's mentioned and seen in a family photo, but doesn't make an appearance.
- Good Parents: Like his wife, he deeply cares for their daughter.
- Unnamed Parent: Even Yoriko doesn't mention his name.
Anna's adoptive maternal relatives.
- Happily Married: They get along amazingly and have light teasing with each other.
- Nice Guy: Both prove to be open-minded and kind.
- Open-Minded Parent: Though they aren't technically Anna's parents, they are pretty laid back. For example, they aren't seen scolding her for staying out late at night or calling another girl "fat pig", but for the last one they do encourage her to offer an apology.
Residents of the Rural Seaside Town
A young girl that Anna eventually befriends.
- Affectionate Nickname: In the English dub, her brother calls her "Sherlock".
- Constantly Curious: Wants to know about the Marnie person.
- Girlish Pigtails: Her regular hairstyle.
- Intergenerational Friendship: She and Anna (both school age children) form a bond with Toichi (a man in his 50s or 60s).
- Nice Girl: Sayaka is a well-mannered, young lady.
- Spoiled Sweet: Sayaka comes from a rich family, but is a nice person.
A lady often seen painting.
- Chekhov's Gunman: Hisako is only seen a couple of times in the movie. However, she turns out to be an important character when she reveals a lot of information about Marnie, which inadvertently leads to Anna discovering she's Marnie's granddaughter.
- Childhood Friends: It turns out she and Marnie were this.
- Cool Old Lady: Hisako is a friendly lady who paints on a daily basis.
- Nice Girl: Polite all the time.
- Nice Hat: Wears a sunflower hat.
- Rapunzel Hair: Her hair reaches to her waist.
A quiet old man often seen fishing.
- All of the Other Reindeer: Downplayed. He's not actively ostracized, but some of the town folk in his home consider him "weird". The worst kind of treatment he gets is some teasing from brats.
- Cool Old Guy: He takes Anna, and later Sayaka, on boat rides.
- The Ghost: In the past history of Marnie. Hearing Anna and Sayaka mention her name in his presence leads to the only time he speaks in the film. This implies that he was present in the town when Marnie lived in the house and remembers her. Though he doesn't actually appear in any of Marnie's scenes and likely only ever saw Marnie from a distance.
- Intergenerational Friendship: Toichi (a man in his 50s or 60s) forms a bond with both Anna and Sayaka (two school-age girls).
- Nice Guy: He's quiet but a good man.
- Nice Hat: A green cap.
- The Quiet One: He only has a couple of lines in the entire movie.
A young man Marnie is often seen with.
- Childhood Friend Romance: Hisako reveals that he and Marnie, both childhood friends, married each other.
- Disappeared Dad: Kazuhiko died when his and Marnie's daughter, Emily, was just a toddler.
- Living Emotional Crutch: Deconstructed. Kazuhiko's death caused Marnie to go into shock to the point she had to be sent to a sanatorium and send Emily to boarding school to be properly cared for. Which ended up straining the relationship between mother and child.
- Nice Guy: He's not seen a lot, but Kazuhiko proves to be a pure gentleman. Which is why Marnie married him.
- Except for the part where, after Marnie confides in him that she is terrified of the silo the maids used to scare her into good behaviour with, his genius idea is to lock Marnie up there alone then act as her saviour when he finally appears to "rescue her".
- Satellite Love Interest: No more information, except him being Marnie's husband, is revealed.
- Small Role, Big Impact: He only appears a few times, but he plays a huge role. He turns out to be Marnie's husband. And his death indirectly caused the estrangement between Marnie and Emily, and later Anna being placed in social services.
A character who has a major impact on the story.
- Break the Cutie: Marnie sending her to boarding school for most of her childhood caused Emily to resent her.
- Missing Mom: She died in an accident and turned out to be Anna's mother.
- Mixed Ancestry: Half Western and Half Japanese.
- Small Role, Big Impact: Only appears in last parts of the movie, but it was her death that caused her daughter, Anna to be eventually placed into foster care.
- Used to Be a Sweet Kid: Implied. When Emily was younger she was apparently happy, but being sent off to boarding school shaped her hatred for her mother.
- Walking Spoiler: Revealing more than her name gives away the entire story.