The basic thing you need to know is that Jake & Leo are Awesome and everyone else is a Loser.
Leo Leshkova is an artist interested in assassination as an art form, murder for its own sake. He is brilliant, curious, athletic, and visionary. He's also a little bit psycho. He was raised by his senile grandfather, Vasili, who taught him what he thought Leo needed to know about the world. Since Vasili is ex-SpetsNaz, this included a lot of unconventional material. Leo's confidence can be appealing, but he's also arrogant. He believes he is relentlessly logical, like Spock, but in fact he's driven by weird passions... also like Spock.
Jake Dannon is extremely polite, intensely concerned about boundaries and competition. Why would such a nice guy follow Leo into this crazy, immoral scheme? For one thing, he needs the money. His father cut him off when he refused to play football any more, so he has to put himself through college in New York City. But beyond that, Jake is really goddamn angry about stuff, and will break someone's face if provoked. It might be that what Jake really needs is a girlfriend, but what he's got is Leo, and that's going to have to do.
Vasili Leshkova is Leo's grandfather. He's healthy as an ox, but his mind is going. He has forgotten how to speak English and often mistakes Leo for Leo's father, Alyoshka. He was a colonel in SpetsNaz, the Special Forces of the USSR. His departure from Mother Russia is shrouded in mystery, but underworld legends speak of a train robbery and a string of murders throughout the 1970s. In the US, his true identity was an open secret to the right people. In 1995 his two sons murdered each other and Vasili raised Leo, his only grandson, by himself. He remains a dangerous man, made more dangerous by the fact that he doesn't really understand where he is. He is sure that people are looking for him, trying to steal the back money that he stole from the Russian army.
Bojack Hamms is named after his uncles Bo and Jack, whom he has never met. He grew up in Baltimore, where he was taught the rudiments of assassination at an early age by a crew of drug enforcers. By the time they self-destructed he had made ties to organized crime outside the city. Bojack thinks of himself as the consummate professional assassin, but he's getting dangerously close to burnout. He probably just needs a vacation.
Lugo Vicente started out working for his uncle, a coyote, smuggling Mexicans into the United States, then graduated to drug-running and kidnapping and eventually became a hitman for Mexican drug cartels. He went freelance, forming a loose partnership with Bojack. They are friends, but have never quite trusted each other. In Lugo's mind, there's only two things worth living for: payday, and an unending list of people to kill.
is an adequate detective but his heart hasn't been in it for a long time. A long career in the NYPD has taught him that Law Enforcement is more about gratifying the ambitions--and bloodlust--of cops and lawyers than administering justice. Oh, he'll do his job, but he's not going to stick his neck out or follow hunches. He's getting too old for that shit.
Detective Javier Colona is not quite as fidgety as a six-year-old. He's had his gold shield for less than a year and is eager to make cases, make a name for himself, and become the Dominican John McClane.
Agent Laurent Bowen was a young uniformed officer in the 1980s when he was recruited by the FBI to infiltrate a drug operation during the crack epidemic in the 1980s. He became a competent but not outstanding FBI officer; he was probably suffering from untreated depression and PTSD from his undercover work. A string of loosely-connected murders, dating back to the mid-80s, consumed his attention, but couldn’t make enough of a case to justify a full-on investigation. Then 9/11 happened, and no one wanted to hear his nonsense about serial killers and assassin’s guilds. Bowen was increasingly marginalized. He’s seen as unstable and out-of-touch by his superiors, many of whom are younger than he is. He is a glorified records clerk in a forgotten back office in New York, but he's got files on everyone. If he could get his act together, he could blow this case wide open.