Genosai

"If this is the path you choose to walk… I will not interfere. But if you turn the people against you… Will you be able to function as a hero? This worries me. But if you ever get in a tight spot… I will be there"

- Genos saying he’ll always support Saitama Genosai is the slash ship between Saitama and Genos from the One Punch Man fandom.

Canon
Saitama and Genos first met when Genos was fighting against Mosquito Girl. Genos notices Saitama standing idly nearby and warns him to leave as it is dangerous, before continuing his fight, when he blasted away an attack from the monster, however, the heat and power from the counterattack was so strong that it burned away all of Saitama's cloths, leaving him naked. Saitama doesn't seem too concerned about his lack of coverage and instead comments that Genos "really saved him" and that he's quite amazing right before Mosquito Girl reappears and attacks Genos. She injures Genos enough that he almost self-destructs, but doesn't, because Saitama steps in.

Genos watched as Saitama defeated the monster with one slap, he was in awe of his power and asked for Saitama to take him in as his disciple, which Saitama agrees, not taking Genos seriously.

A week later, Genos shows up at Saitama's doorstep. Saitama is annoyed and wants Genos to leave, but Genos is persistent and ends up telling Saitama his entire life story, which Saitama has made clear he has no interest in listening. Genos credits Saitama for saving him and keeps talking until Saitama snaps at him in annoyance to shorten whatever he's saying.

Genos does shorten it, and Saitama agrees to take Genos as his disciple seriously this time. They are interrupted when some monsters from the House of Evolution burst through Saitama's ceiling. Genos goes to take out the two outside but Saitama does it before he can. More monsters appear, when Saitama is faced with of the monsters while seemingly helpless, Genos is concerned and is momentarily distracted from his battle with Armored Gorilla, they both end up defeating their respective opponents and head towards the House of Evolution.

While Saitama was annoyed by his new disciple at first, he gradually warmed up to him. He also hold respect for Genos as he's an S-Class Hero and also his overall abilities. Saitama doesn't seem too worried about Genos' body safety though, as he's almost completely destroyed in every fight. Even though that is the case, Saitama still takes care of him should be torn to shreds or just saves him from complete destruction.

They're currently roommates and officially teacher and disciple, even though that mostly comes from Genos' side. However, Saitama often gives good advice to him. Genos has an incredibly amount of respect towards Saitama, as he knows he’s surprisingly strong, but also since he saved his life.

Monster Association Arc
After Saitama finds out the secret tunnel ang gets to confront Orochi, the two of them didn’t got any interactions together as they were on completely different locations, until chapter 156, where Saitama was teletransported by Blast into the battlefield with the other heroes, they finally got to reunite. Garou was hunting down the whole Hero Association. Instead of fighting him, he just ignored him and went straight to check on a destroyed Genos. Saitama asks if Garou was the one who wrecked him with a notably angry face, but Genos told him he wasn’t. King arrives and tells Saitama Genos fought well, so Saitama smiles and congratulates the cyborg. Genos feels a little bit sad since he believes he didn’t become stronger, as he wasn’t able to self-destruct. The Caped Baldy says he doesn’t understand much of that topic, but puts his hand on Genos’s core and tells him that if he wasn’t able to do it, it means that his core has become stronger. Genos stares surprisingly at his Sensei’s hand, but then smiles and thanks him and Dr. Kuseno for everything. Saitama smiles back him and bashfully blushes as he tells his mentor he hasn’t do anything.

Songs
Hoshi Yori Saki Ni | Hiroko Moriguchi
 * This ED of the first season of One Punch Man, have lyrics that some would claim sound quite a bit like something Genos would sing to Saitama, and this is a love song.

Chizu ga Nakutemo Modoru kara | Makoto Furukawa
 * This ED of the second season, sang by Makoto Furukawa (Saitama’s VA in the Japanese dub), has lyrics that are speculated by some to be a reply to the first ED.

Quotes
"If I continue being a hero, this won’t be the last time someone’s out for revenge against me. I just have to believe there’ll be people in this world who appreciate all the things I do. Oh yeah, There was that guy who wanted to be my apprentice…"

- Saitama remembers Genos "Even if the public doesn't appreciate you, I will still follow you"

- Genos thinking about Saitama "Uh-oh, if my outfit gets torn up, (the core) will just fall right out. I better keep a tight grip on it…"

- Saitama wanting to protect Genos’s core

Fanon
Their alternate ship name "EggToaster" or "Egg x Toaster" comes from Saitama's egg-shaped head and Genos' arms, which has vents that somewhat resemble a toaster's slots.

In the fandom, the ship is usually a bromance or platonic, but because there is still a decent about of SaiGenos shippers who ship it romantically, the term is usually referring to the romantic pairing. In fanworks they are often depicted in a domestic lifestyle, often making fun of Saitama's penny-pinching tendencies. Fans have also noticed how Genos rarely shows his emotions or anger unless Saitama is involved, usually when someone insults or "endangers" his master. Due their age gap (Saitama being 25 and Genos 19), some fans just leave them as a friendship, and also since Saitama hasn’t show to have any romantical attraction to a gender in general.

SaiGenos is the most popular and one of the oldest ships in the One Punch Man fandom, as well as the most written ship for the fandom on AO3.

Fandom
FAN FICTION
 * Saitama/Genos tag on FanFiction.Net
 * Saitama/Genos tag on FanFiction.Net

TUMBLR

WIKIA

Saitama’s relationship with Genos on One Punch Man wiki

Genos’s relationship with Saitama on One Punch Man wiki

Trivia

 * Genos is notably one of the only people Saitama seems to genuinely care about, and he is possibly the person he cares about the most.
 * Genos is also one of the few people who knows of Saitama's strength and became his friend.