OttoKallen

OttoKallen is the het ship between Kallen and Otto from the Honkai fandom.

Canon
ELAN PALANTINUS

1465 in Anno Domino the 13 years old Otto meets Kallen for the first time. His invention, a small airplane, flew over a wall. There it was found by Kallen and brought him back. Kallen praised his invention, which made Otto very happy. Because he was very ill and fragile, he was of little value to his family. Kallen was the person who first recognized him and his efforts. They became very good friends.

At some point (timeline is unclear) the children were attacked by Honkai beasts. Kallen's father, Francis Kaslana, protected her with his life and died from burns from his weapon Shamash. It was then that Ottos realized for the first time that if Kallen were to follow her father's path and become a Valkyrie of Schicksal, she would die the same way. This fear even haunted him into adulthood.

1470 Kallen is already a Valkyrie of Destiny. Her Kaslana oath is put to the test for the first time as her opponents are not Honkai beasts but humans. she decides to fight her opponent "The Heavenly Immortal", but loses because she lacks the necessary motivation. It is Otto who runs across the battlefield and stands protectively in front of Kallen. But her opponent lets her live. Destiny loses the war against Shenzou.

In 1475, after losing to Fu Hua, Kallen and Otto saved a girl in Geneva. They both give her a name Vera.

In Gun Girls Z 2 Era:ZERO Otto und Kallen are married in one tmeline.

Quotes
"Otto… Do you still dream of making a flying machine?"

- Kallen

"To save the one and only her, I can only create new possibilities in the past. The other future will belong to her…"

- Otto

Fanon
OttoKallen is popular mostly in the Eastern fandom. The ship is mainly rivaled by SakuKallen and OttoWelt shippers. It's also common to see shippers enjoy this ship along with SakuKalpas.

The ship is beloved for it following the childhood friends to lovers trope and having one of the saddest story in Honkai fandom.

Some people draw parallels with KiaMei due to similarities of the two ships.

Trivia

 * In the official description of “Elan Palatinus” is "Year of our Lord 1470, Schicksal rallied the princes of Europe and unleashed a Crusade to conquer the Far East, only to be defeated by an immortal warrior. Kallen Kaslana, a powerful Valkyrie warrior, must make a choice between love and her oath". Since Yae Sakura was not mentioned in the manga, the sentence "to make a choice between love and duty" is about Otto.
 * In the box with the letter to Otto, Kallen Kaslana left two roses, which according to Chinese traditions are considered a sign of apology between lovers.
 * In “Thus Spoke Apocalypse” before little Otto and Kallen were shown sleeping by the tree, there were two bright stars, Altair and Vega. Old Chinese legend tells of the forbidden love story between the goddess Zhinü (symbolized by the star Vega) and the cowherd Niulang (symbolized by the star Altair).