Void Fist siblings

Void Fist siblings is the family ship between Suiko and Suiryu from the One Punch Man fandom.

Canon
Suiko and Suiryu are siblings, Suiryu being the older brother in the dynamic while Suiko being the younger sister. They both were trained to the Void Fist martial arts style by their grandfather Suicho. Though Suiko seemed to be more disciplined than her brother as she was the one to take over the dojo after their grandfather passed away. Even though the responsibility should have been Suiryu's as apparently their grandfather would have wanted that. Meanwhile, Suiryu lived a carefree life and fought because it was fun and went on dates with young women.

Monster Association Arc
Suiko is first time seen in the canon storyline when she arrives to the hospital to check on Suiryu after the horrible Monster Association raid attack in the 22nd Super Fight Tournament where Suiryu got injured very badly. While Suiryu is asleep, Suiko teases her brother and he wakes up. They have a conversation about what has happened while Suiryu was living his carefree life. Suiryu reveals that he wants to become hero for real, after Suiko pointed out he had also wanted to be a model and an actor. Suiko then starts to peel an orange for Suiryu, but they are interrupted by the news showing Tatsumaki's giant light spear that's drilling to the ground.

In later chapters, when Garou and Metal Bat were starting to team up to take on Sage Centipede, the news would showcase the moment when "a vicious monster" (Garou) jumped on the news helicopter and scared the crap out of them, making them retreat. They do point out that they survived without incident. Suiko got nervous about the monster, but Suiryu points out that he doesn't feel any killing intent from it, which confuses Suiko.

Webcomic
The webcomic storyline is a bit different compared to the manga.

In the webcomic, the reader meets Suiko (and Suiryu) for the first time after the Monster Association Arc. The Hero Association contacts her, hoping that her grandfather would have been interested in joining their Association to replace the retiring S-Class hero, Bang. However, Suiko tells them that her grandfather is dead, but then ends up joining the HA herself. She mentions to them that she has no idea where her brother is as she has not seen him for years. She doesn't seem to have high hopes that he is doing anything serious and is still having fun somewhere.

The siblings eventually meet again after a long time when Saitama had left an impression to Suiko and she is talking with him outside the Hero Association. Suiryu has joined the Neo Heroes and he is happy to see his little sister. He tells her how surprised he was to hear that she joined the Hero Association, while Suiko didn't know there were other places that hired heroes as she was surprised to hear her brother is a Neo Hero. However, she starts to tear up as she is glad that her brother finally is taking things a bit more serious and has a proper job.

Suiryu offers to introduce her sister to the Neo Heroes so she can join them instead as well, because apparently the Hero Association is already a failing business. But Suiko refuses as she tells him that's not true. She then asks for support from Saitama, who had been standing right there. Suiryu is confused who Saitama is (as in webcomic they have not met yet). Suiko tells Suiryu that Saitama is far stronger than he is. Suiryu misunderstands this as Saitama being the one Suiko is interested romantically, because their grandfather used to tell Suiko that her future partner should be one who is stronger than Suiryu and their grandfather. Suiryu then suddenly kicks his leg towards Saitama's face, but he doesn't even flinch when Suiryu stops it right there. He doesn't believe Saitama is stronger than him, but if Saitama is working to become a shining star of a hero, he will allow him to date his sister. Outraged, Suiko denies her brother's misunderstanding as Saitama is actually the one who saved her life and nothing else.

Suiryu tries to get Suiko to join the Neo Heroes again and also extends this offer to Saitama as well, but they both refuse it. Suiko also tells Suiryu she wouldn't want to use such a lame Neo Heroes suit like him and Suiryu is surprised to hear his sister thinks the suit is lame. He reveals he is rather fond of the suit and asks her if the design is really that bad. She confirms it. This shuts up Suiryu for a second, but then he tries to sell the idea of the suit more to her by telling it has amazing features and she should definitely try it on. Suiko stops him by telling him to go home.

Fanon
The Void Fist siblings is liked family ship in the fandom as the siblings seem to have fairly warm relationship and their dynamic seem to be rather fun. Unfortunately, not many fan works are being created about them at the moment due to their limited interactions being mainly in manga and webcomic only. However, there is hope that the anime or future manga chapters will bring more content so the ship will get more artworks and fanfics in the future.

Fandom

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