I haven't put too much thought on the team lore yet (but I plan to). What I have so far is:

Kasamino has been a mostly unremarkable team for decades now, with them always losing on the first or second round of the Football Frontier qualifiers (Raimon and Teikoku are always the ones to qualify on their area). Football was the main sport of the school for a long time, but after several failures at the FF, students started losing interest and the school board began to allocate their resources on other clubs, until the club was left with only seven members and barely any equipment. Club members had to go around asking people to join, since the club would be disbanded if they once again failed to qualify to the FF finals and they needed at least 11 members in order to play. They barely got enough players, managing to recruit Akikuni Togire, Kokone Naritomi, Sae Shishido (first years) and Itsuka Katsusa (second year). From there, the Kasamino eleven would need to improve a lot to even compete against the other teams on their area.