There are multiple characters in the game.
Betty is introduced in the beginning of the story. Betty is Roscoe's wife/secretary. Betty helps Johnny through the missions. Johnny falls in love with Betty because she has a nice a** and big breasts. Betty is assaulted while in the slaughter house. Players can switch between Betty and Johnny after all the missions have been completed by going to the blue arrow outside Betty's apartment. Betty is seen as one of the sexiest people in the story.
Roscoe is a creepy character that lives in the swamps, afraid he will be arrested. Clive Klick tells the player to meet Roscoe first. Roscoe barely helps the player, he only gets him in in trouble with a Hispanic gang run by Victor Vega. Roscoe does not go to a hospital when he has a heart attack, afraid he will be arrested. Betty then beats up Roscoe but does not kill him. Afterwards, Johnny leads the police to Roscoe's hideout.
L.C is first encountered when Betty, Joey and Johnny are trapped in the slaughter house. When Betty is almost raped she beats up the raper and takes his cell(US)/mobile(UK) phone and calls Johnny's friend, L.C. L.C. steals a helicopter from the police at the airport and flies to the slaughter house, killing the guards and helping Johnny, Betty and Joey escape. L.C. later helps Johnny through the missions. LC is originally from Baghdad and is one of the two links between both Gangstar games (West Coast Hustle and Miami Vindication, the other link is the mentioning of Eddie Fallon by Amelia).
Herman is one of my favorite characters. Herman is first seen when Johnny gets a bouncer job and now works for him. Herman then sends Johnny to do jobs for him involving going into Jones' Turf. Johnny then gets an important name: Reginald. Herman then gets killed off by Johnny because Herman led them "led them down the road to ruin" (quoting Betty's words). Herman is a rich business man / gang leader.
Victor Vega, the big drug dealer of Miami, owns a car showroom in front of his house. He is also close to the cops, and has been co-operating with Roberto Bolano (a police chief), who Johnny kills later in the game. Vega's hotel was supposed to be protected by Officer Ramirez, but then Ramirez runs to see what happened to Roberto Bolano because he wasn't copying. The police arrest Victor Vega for killing Bolano (even though it was Johnny) because Johnny bribed a police detective to leave him alone and in the process claimed that Vega killed Bolano. Later in the game, Johnny does not admit to killing Bolano when Joey queries the police force's decision to arrest Vega on his murder, instead he says "Bolano probably got on the wrong side of some mean, dirty b*stard, but the cops just grabbed Vega to pin it on". It is unclear as to why he said this.
Albert is first seen in the introduction. After Johnny leaves his car, his car gets blown up. Johnny, angry over the loss of his car, decides to take one of Albert's car thus starting the fight between the two. He is later seen to get revenge on Johnny, however, decides to get it over after johnny helps him get his "new" car. Johnny and Albert often race together.
Joey is Johnny's 15-year-old brother. He was kidnapped by Clive Klick whilst walking at his home city (unknown) with Bobbie Jean (believed to be Joey's girlfriend). Joey's kidnapping is the reason that Johnny came on his journey down to Miami where he met up with Betty and went through Roscoe, Victor Vega and Herman Kaye to find him. Joey is found at the end of chapter 2; in chapters 3-5 Johnny gets revenge on Roscoe, Victor Vega, Herman Kaye and Monique Jones.