Sumibin

"Oh, Sumia, I'm so happy! It's like we're in a storybook of our very own."

- Robin to Sumia, in Awakening

Sumibin is the non-binary ship between Robin and Sumia from the Fire Emblem fandom.

Canon
Robin is an amnesiac tactician and Sumia is the clumsy pegasus rider in training and member of the Shepherds--guard of the kingdom of Ylisse. Robin and Sumia first meet when he is accepted and introduced to the group, in the shepherds base. At that time, Sumia had a crush on their leader, Prince Chrom, and when introducing herself she trips and falls, making herself ashamed.

The couple can be first paired when they go to Ferox in a diplomatic mission that goes wrong. Sumia that was left behind, then appears to save their leader, Chrom from being arrowed, and so Robin creates a strategy to divide themselves into pairs to win the battle against the Ferox soldiers.

From that day, they start to bond over their interest for books. Robin likes technical, non-fiction stories while Sumia is more interested in romances and fantasy. She feels frustrated when when all of her suggested books don't catch Robin's interest. Later, Robin reveals that they quite enjoyed the books suggested by her and they both lament when a good book comes to an end. Eventually Sumia confess that she likes to read and roleplay as fictional characters to escape reality, Robin finds it a bit weird but is ok with that, they tell her that she doesn't need to pretend to be someone else because she is perfect as she is though.

Sumia eventually realizes that her extreme escapism was not healthy. If Robin is male, he can confesses that he couldn't stop to think on her and had fallen in love. He says that he is not the fairytale prince of her dreams but proposes to her. Sumia accepts by saying that by being together they can make their own book story. They marry and eventually have two daughters, Cynthia and Morgan.

Robin discovers that he is actually son of Validar, the king of Plegia, a bordering kingdom that's enemy of Ylisse, and that he was raised to be the vessel of the dragon Grima, the dark deity that would bring the destruction of the world. They can possibly find their time traveling daughter Cynthia who came back from the apocalyptic future, and also their  amnesiac younger daughter Morgan who just have memories of Robin but eventually she has lapses of her sister Cynthia and mother Sumia from the original future.

It's eventually revealed that Robin is the one that killed the exalt of Ylisse Chrom, and that Sumia also died in the war that happened after the assassination of the king. When Sumia was killed, her Pegasus made her way back to Cynthia, but it's not known how exactly she died, nor how was her life in the time period which Robin was possessed by Grima in the original timeline, the only information provided is that Sumia and Robin to some extent raised both daughters to the point of them having happy memories of their parents.

Children
Cynthia 

Cynthia is a pegasus rider, one of Lucina’s allies who came with her to Ylisse from the future. Like her parents she shares a taste for stories, and she grew up hearing the feats her parents and the other Shepherds. Unlike the shy and timid Sumia, Cynthia’s a hyped-up action girl who dreams of fighting for justice, just like the heroes she heard so many stories about. She’s so obsessed with the stories that she starts off her battles by making a dramatic and heroic entrances.

Morgan

Morgan aspires to be a tactician like her father, Robin. In the version of the future that is gone wrong, she came from was a world of despair, where Grima had possessed Robin and made a servant of Morgan. Morgan, wishing to become Grima’s pawn, wrapped herself in an aura of darkness. She battled Chrom and his friends,and possibly her own sister and mother. Morgan only embarked on this path because she couldn’t separate what Robin had become from the gentle father she once knew. When she goes back to Ylisse in the current timeline, she loses her memories.

Fanon
It's the second most popular pairing for Sumia, and a fairly popular ship for Robin. Robin and Sumia can canonically marry each other only if Robin is male, since the option is not possible with female avatar. But the femslash version the ship also has a fair amount of followers in the fanon.

In Awakening, Tiki hints that Robin is a similar person to the hero king Marth, who married the pegasus knight Caeda. Sumia is often likened to Caeda due to also being a pegasus knight. Therefore, due to the plot twist of Robin being similar to personality to Marth instead of his direct descendant Chrom,  some point out that Robin x Sumia may be a reference to the pairing Marth and Caeda.

Fandom
FAN FICTION
 * The tactician and the Maid of Flowers, a C2 community for Robin/Sumia fanfiction on fanfiction.net
 * The tactician and the Maid of Flowers, a C2 community for Robin/Sumia fanfiction on fanfiction.net

Trivia

 * Robin is the only possible father that can give the hero class to Cynthia.
 * In a cutscene when Robin is reminiscing the past and first time he met the shepherds, he reminds of the first time he met Sumia and laughs thinking about her.
 * The skill exclusive to Robin's class, tactician, Ignis, shows petals of red flowers in the background. In Cynthia's conversation with her father she makes a surprise to him with petals of flowers in the battlefield.