OttoKallen

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

Canon
Otto and Kallen are childhood friends. They both have different views, however they both want to understand each other. Otto says that Kallen is love of his life and that his heart also died with her death. In Houkai Gakuen 2 (Girls Guns Z) they were married.

Otto and Kallen met each other when they were young. As a child, Otto was not supported by anyone, as he was very weak. To compensate for this, he began to develop his mental abilities, but no one noticed his efforts. Until a girl named Kallen Kaslana burst into his life. She gave him hope.

When Kallen lost her father, Otto asked the Void Archives for help and wanted to resurrect Francis Kaslana by giving his own life. Kallen didn't want to lose Otto so she was able to stop him.

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

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).