The terms below are a selection of common euphemistic terms for various types of ninja. I’ve codified them into a structured rank system rather than use the simple Genin-Chunin-Soke-Jonin standard rank system that was historically used to give a more elaborate feel in my games.
Mitsumono - A system of warriors who specialize in gathering information through observation.
Yaburi-suppa - A type of brigand from western Japan.
Kikimonoyaku - A person who gathers enemy secrets by listening to rumors and gossip.
Kakae-suppa - A type of thief from western Japan.
Kobushikata - agents who gathered information by performing manual labor such as road repair and construction.
Nokizaru - A person who specializes in entering houses through the roof, lit. "rooftop monkey"
Rappa - A system of warriors who specialize in gathering information, specializing in collecting verbal information.
Nusumi - Warriors skilled in special warfare gathered together in bands called nusumigumi ("thieves band") for a specific goal.
Kusa/Kusamono - "Grass" or "grass person", a term for an agent who "hides in the grass." that is, agents who hide in the fields and forests, or in peasant houses.
Musokunin - A type of warrior who specializes in one of five aspects of war.
Kansai - A person who searches for detailed information.
Genin - "Low man," a rank and file member of a ninja clan.
Sekkou - A term for a scout or spy.
Satoiri - A ninja in the homeland (sato) acting undercover in the various han of rival lords. They monitor dissident han and gather evidence that could be used to blackmail or dissolve a han when it stepped out of line.
Akeyashikiban - Warriors assigned to guard a castle until the shogun or daimyo returned.
Sokkan - An espionage agent who performs assassination.
Oniwaban - "Guard of the inner garden," a term for ninja who adopted roles as the gardeners and landscapers of the shogun, but were really bodyguards.
Shinobi - A person who provides special services, such as information gathering and passing secret messages.
Okashira - A senior oniwaban.
Kanja/Kancho - A term derived from Chinese to refer to master spies.
Onmitsu - Government officials who covertly gather information.
Chunin - "Mediator," A ranking ninja such as a senior agent or instructor, who deals with the day-to-day operations of a ninja clan.
Soke - "Family head," the officer who acts as the visible master of a ninja clan, in reality being the mouthpiece of the clan jonin.
Jonin - "Standing office," the true head of a ninja clan, though not seen by the rank-and-file members of the clan.
Kunoichi - The term for a female ninja operative.