Saitama might be the strongest character in One-Punch Man by far, but he's surrounded by a wide variety of organizations, groups and factions. As One-Punch Man begins, Saitama is a hero just for fun, beating up monsters without pay or recognition, and becoming increasingly frustrated with the lack of challenge his adversaries present. As Saitama opens up to the world of professional heroism, he encounters an ever-growing cast of colorful characters, both good and bad, and many of these operate within teams.
Each of these collectives has their own philosophies and traits. Some are brought together by their moral alignment, others their fighting technique, and some simply because their dress sense matches up. Many of the larger groups even have subdivisions of more specific fighters that have joined behind a common cause.
The Hero Association - The official vigilante association Saitama works for. Split into classes S, A, B and C, the Hero Association is funded by wealthy backers and send their own team of verified heroes to handle a variety of threats. Although essentially a force for good, the Hero Association has made some questionable decisions in the name of cold, hard cash.
The Blizzard Group - Within the Hero Association are several groups of fighters that work as a unit, and the Blizzard Group stand at the top of the B Class as gatekeepers to the next stage. Led by the psychic hero Fubuki, otherwise known as the Blizzard of Hell, the Blizzard Group total around 30 members and have various abilities, but all possess some level of telekinetic ability thanks to their leader. Fubuki is fiercely loyal to her subordinates.
Tank Top Army - Another subdivision of the Hero Association, the Tank Top Army are united by their love of sleeveless tops and call the S-Class Tank Top Master their leader. Unlike the Blizzard Group, the Tank Tops are scattered throughout the classes of the Hero Association, and mostly rely on their brute strength in battle. They believe tank tops to be the root of their power, but are routinely beaten.
The Surly Brothers - This trio consists of 3 siblings that use emojis as their superhero gimmick. Consisting of A-Class Smile Man, B-Class Crying Man and C-Class Angry Man, all three are background members of the Hero Association and use giant, deadlier versions of traditional toys in battle, such as a spinning top and a kendama.
Bang's Dojo - Reduced to only a single member thanks to Garou's campaign of fear against his former master, the S-Class Bang trains his own team of martial artists in a dojo specialized in the way of the Water Stream Rock Smashing Fist technique. Only Charanko remains loyal to his master.
The Swordmasters - Although not strictly aligned with the Hero Association, the Swordmaster Council includes the S-Class Atomic Samurai and a handful of other elite swordsmen who form their own separate unit against dangerous threats and monsters. Atomic Samurai himself has a separate set of disciples who are ranked within the Hero Association - Iaian, Bushidrill and Okamaitachi.
The Monster Association - The polar opposite of the Hero Association, "Gyoro-Gyoro" established this team of supervillains by installing the formidable Orochi as the leader, and actively recruited those who were already monsters, as well as strong fighters who were willing to turn. The strongest villains are known as Executives, and rank higher than the remaining fodder.
The House of Evolution - Early antagonists to Saitama, the House of Evolution was set up by Dr. Genus with the intention of creating the perfect living creature through science. Monsters such as Mosquito Girl and Armored Gorilla all hailed from the House of Evolution, but were easily handled by Saitama or Genos. After seeing his experiments crumble before Saitama's modest training regime, Genus decided to give up his mission.
The Paradisers - Briefly appearing in One-Punch Man's first season, the Paradisers envisage a world where no one has to work, and they're willing to fight for that dream. Preferring power suits over genuine fighting skill or special abilities, this group are easily dealt with by Speed O' Sound Sonic. The Paradisers are easily identifiable by their baldness - a point of contention for poor Saitama.
The Organization - The faction simply known as The Organization is a coming together of various powerful robots in order to collate data and improve their designs. Free from moral constraints, The Organization later align with the Monster Association, albeit probably just as a means to an end.
One-Punch Man season 3 is currently without a release date. More news as it arrives.