Racketeer - a criminal who engages in "protection racket" scheme. Where you go around town and demand businesses to give you money for "protection".
These days, this job is mostly taken over by mafia, so if you try it on some mafia's territory, you'll have to fight them.
@mysticbluedragon - at some point earlier, they said that tamers get a new slot every 20 levels. Onii-san hit level 20 recently, and has been looking for a new monster.
But they still need an empty place in the party to be able to actually field their summons. I don't remember if the party limit was ever mentioned in this game, but it's probably 6. If so, it means that with a full set of lolis, MC can only deploy 2 summons, and with just the twins - 3 summons.