We never actually see any of these guys on the pages of Kim, but they play an important role in the plot nevertheless. When Mahbub Ali first tells Kim to bring a message to the then-anonymous Englishman in Umballa (who later turns out to be Creighton), it's about the rebellious plans of the Five Kings. These Kings all rule small areas on the border between India and Afghanistan, but they owe their loyalty to the British state. When Mahbub Ali confirms that they are making plans against the British Empire, Creighton immediately sends eight thousand troops up the border in case they decide to go to war.
Later on in the novel, we find out that nothing came of this first troop deployment. It seems like the Five Kings have backed down. The British Indian State has even given money to two of the Five Kings, Hilas and Bunnar, so that they will hold their borders against the three remaining Kings if they rebel and try to invade northern India.
But the Babu gets word that these bribes haven't been enough. Hilas and Bunnar have been in secret talks with the Russian Empire to break off ties with Britain, so the Babu drafts Kim for help in getting proof that Hilas and Bunnar are trying to double-cross the state, leading to Kim's first real mission as a Secret Service agent. It's thanks to the plans and schemes of these offstage Kings that Kim finally gets to prove his worth as a spy.