As is the case with any long-running series of books or films, the main characters are introduced in the first installment only for other interesting ones to debut a bit later on. The Hunger Games falls into that category, especially given the way that the world expanded as the story progressed.
After the first film focused on Katniss and Peeta in the games, the sequels gave more time to existing characters like Gale and Primrose. It also debuted other competitors in the games, as well as more villains and even characters who were fighting to rebel against The Capitol.
Pollux
Not appearing until Mockingjay - Part 1, Pollux is a character who makes a decent impact. He appears as someone working with Cressida as a cameraman who films Katniss in her adventures against the Capitol. What made Pollux stand out was his status as an Avox.
Being an Avox meant that Pollux's tongue was cut out due to some of his acts of rebellion against the Capitol. In the books, his relationship with brother Castor was more pronounced. Without it being as prominent in the movies, Pollux still managed to stand out and it's thanks in part to the performance of Elden Henson.
Cressida
Another character who didn't debut until the first Mockingjay installment, Cressida proved to be one of the most loyal people towards Katniss. She joined the rebellion with her camera crew and was often at Katniss's side whenever she went out in public. One could argue that she was a secondary bodyguard behind Boggs.
Cressida and Katniss bonded a bit, though it wasn't truly given as much screen time as it deserved. She is one of the few characters who made it out of the series alive, at least as far as fans know. Her final appearance came at Snow's execution, which was being filmed.
Boggs
This film series featured some great actors but only a few have been successful enough to win an Academy Award. Mahershala Ali has pulled off that feat twice since appearing in this franchise as Boggs in both Mockingjay parts. He was President Coin's second in command.
Despite his close relationship with Coin, Boggs seemed to see beyond it and ultimately had Katniss's best interest at heart. He took his role as her bodyguard seriously and the two grew to be fond of one another. His dying words saw him give Katniss a pivotal warning about who she should trust.
President Alma Coin
When Alma Coin first appeared, she seemed like she could represent a beacon of hope for the heroes. Dubbed President Coin, she was the person behind District 13 and seen as a leader of the rebellion. On paper, she should've been one of Katniss's greatest allies in the fight against the Capitol.
Though she was in some ways, she also had her own personal agenda. Although Coin was against the Capitol, she was more interested in her own path to power. She was behind the bombing that tragically killed Primrose, leading Katniss to assassinate her. Coin was a different kind of antagonist from Snow and was played expertly by Julianne Moore.
Plutarch Heavensbee
In The Hunger Games, Seneca Crane acted as a secondary antagonist. While he wasn't pulling the strings like President Snow, Crane was the Head Gamemaker, meaning his actions directly impacted Katniss and Peeta in the arena. His failures led to Plutarch Heavensbee replacing him.
Plutarch first appeared in Catching Fire and seemed to play a similar role as he followed Snow's desires. However, the twist ending of that installment saw him be one of the people behind the rebellion. Plutarch always believed in Katniss even when Coin didn't. It's a true shame that Philip Seymour Hoffman passed away before completing filming.
Mags Flanagan
The concept of Catching Fire allowed for some of the series' best characters to arrive. With the arena being filled with winners from the past, it meant that Peeta and Katniss would be joined by some interesting people. Among them was Mags Flanagan, an elderly winner from District 4.
She volunteered for the games to save Annie Cresta, the young girlfriend of Finnick Odair. That made the two into allies and Mags proved to be as sweet as they come. She was a friend to Katniss and she sacrificed herself for the good guys. Had she lasted longer, she would've certainly fought for the rebellion.
Beetee Latier
In terms of pure intellect and wits, it's hard to top Beetee Latier. Also introduced in Catching Fire, he was a victor from District 3, who won his games by outsmarting everyone. Almost immediately, he proved to be one of the most valuable allies that Katniss ever made.
In the arena, his mind was a huge benefit that helped keep Katniss alive. Following the events of those games, Beetee stuck around to fight for the rebellion, using his brains to help hack into Capitol feeds and cause all sorts of problems. In the end, Beetee was also one of the past victors to vote against the idea of another Hunger Games.
Johanna Mason
Considering Katniss and Peeta were going to compete again in the arena in Catching Fire, most of their opponents felt like antagonists. Johanna Mason, a District 7 winner, instantly rubbed Katniss the wrong way with her attitude and her penchant for stripping in front of Peeta.
In the arena and as part of the rebellion, Johanna continued to give Katniss grief. It made her a unique character because she wasn't afraid to call out Katniss for her actions even if she was on the good side. Johanna suffered torture at the hands of the Capitol, so her touchy demeanor by the final film makes a fair amount of sense.
Finnick Odair
Along with Mags, Finnick Odair was the other District 4 winner. He stood out as someone Katniss wanted no part of because he came across as cocky. Once they got into the arena though, Finnick displayed that he was a true team player who had her back at every turn.
Finnick was a huge part of the rebellion and forged a tight bond with Katniss through shared experiences. He may have flirted with her at first but he ended up as one of her best friends. His death helped save everyone and it's a tragedy that he never got to meet his son.