From Up on Poppy Hill is an underrated gem from Studio Ghibli that brings something unique to the table. It featured student activism in the aftermath of World War II, as well as young love. The romance between the two main characters, Umi and Shun, is heartwarming, yet it could be confusing.
A lot of fans are wondering if Umi and Shun are truly related. After all, in the film, it was stated explicitly that they could be siblings. Despite their budding romantic feelings for one another, it came to the point that they both tried to repress it. But how can this be? And what is the truth?

Umi and Shun Have the Same Photograph
During one of their friend’s going-away celebration, Umi opened up about her father, Yuichiro Sawamura. Her father went out to the sea, and was killed; yet Umi still raises the signal flags everyday. She showed Shun the photograph she has of her father, together with two other naval men.

Shun was stunned as he had the same photograph too. The photograph was stated to be from his father. This affected Shun, as he started to believe that he and Umi might be related. This is why he started to avoid her, in order to prevent his feelings from growing. This bothered Umi greatly, and when she confronted Shun, he showed his own copy of the photograph. He stated that Yuichiro was his real father. Umi followed suit and she started to repress her feelings too.
Despite the situation though, they still cannot help their romantic feelings for one another. Umi confessed to Shun later on in the film, which he reciprocated.
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Shun Was Registered As Yuichiro’s Son in Legal Papers
Shun asked his adoptive father, Akio Kazama, to clarify whether or not Yuichiro Sawamura was his actual father. Akio then shared that Yuichiro showed up to their house one night, with a family registry paper and baby Shun in his arms. At the time, the Kazamas are struggling with the loss of their baby, and immediately took Shun in.

Yuichiro explained that he and his wife cannot take care of Shun, yet Akio insisted that Yuichiro was a good man. Since Shun is recognized legally as Yuichiro’s son, this could have been the confirmation of his relation with Umi. However, there is still a plot twist along the way.
Umi’s Mom Tells the Truth
The answer has been given by Umi’s mother, Ryoko Matsuzaki. Ryoko came home after studying in the United States, much to Umi’s joy. Umi then opened up to her mother about Shun, in which her mother then told her what really happened.
Umi’s parents, Ryoko and Yuichiro were both struggling back then, since they have just eloped. Yuichiro was out at sea, and Ryoko was balancing being pregnant with Umi and studying. Yuichiro then came home carrying an infant Shun one day.

It turns out that Shun is actually the biological son of one of Yuichiro’s fellow sailor, Hiroshi Tachibana. Tachibana was also included in the photograph with Yuichiro. Tachibana was killed in an accident with the repatriation ship, and his wife (Shun’s mother) died in childbirth. Shun’s other relatives were killed during the bombings, therefore leaving Shun with no one.
Yuichiro does not want to leave Shun in an orphanage, so he decided to register Shun as his son in the family registry. However, since both Ryoko and Yuichiro cannot take care of Shun, they decided to give him away to the Kazama family.
Despite this, however, Umi still have her doubts. This is because Shun is the splitting image of her father. This prompted her to break down, finally releasing all of her emotions to her mother.

Confirmed By Yuichiro and Tachibana’s Fellow Sailor
In the photograph, Yuichiro and Tachibana were accompanied by another sailor, Yoshio Onodera. Yoshio is the sole survivor among the three of them. They were close friends, and there was not a day that Yoshio was not thinking of them. He felt tearful being able to meet Yuichiro and Tachibana’s children, it was as if he was seeing his friends again.

This is the final and resolute confirmation that Umi and Shun are not related at all. Circumstances just happened that made it seem like they are siblings. In truth though, despite Shun’s striking resemblance to Umi’s father, they are far from being siblings. They are both the children of sailors who were close to one another, and therefore, they can become a couple.