This category is for popular characters, these characters are popular enough to get larger roles after they're introduced. Here are some people who are considered this
Bender as an antihero and get funny lines was obvious. While an terragonist in the The Beginning, He went on to become one of the main characters in every other story following with his conflicts with archenemies Joker and Discord, other foes , teaming up with The V Team and getting 3 spinoffs for himself as leader of characters in M.O.D.A.B and new charcters. By the end, he is one of the 2 main characters of the series alongside Skipper as seen below.
Lizbeth was not at first a main character but from Slade Strikes Back she became a main character and got a good amount of character development that made her a very skilled heroine. This got to the point of being the main protagonist of The Great Time Travel Adventure and being the sole main hero of the story
Boomer is one of the most promient characters, and he has got a major role in 3 of 4 stories. his first one dealing wtih evil, Slade Strikes Back his developing relationship with Nina and The Great Time Travel Adventure with his relationship with Nina is tested and he falls in love with Lizbeth as a result of this.
Skipper was a small character in Slade Strikes Back, From The V Team Island Adventure on he became a main character and served as Bender's right hand man and confidant. His role has been emphazied to the point of he and Bender are the main male protagonists of The Great Summer Season Trek and Totally Mobian Spies. Also he and Bender are basically the series's main protagonists by the end of The4everrevvial's series
Marceline was always a normal player in her debut story and was a major character when the true villain was revealed. She was liked so much that she became the deutragonist female lead after Lizbeth in The Great Time Travel Adventure and played off as Bender's sidekick against his archenemy as she has a history with Joker as well
Nibbler appeared in just The Great Time Travel Adventure but he had a major role as the second Futurama character to appear, His tendecies made him well liked as he decided to stay with the heroes and is a major character in other stories
Heloise just like Nibbler appeared in The Great Time Travel Adventure, but like Marceline and Lizbeth has a big role as Lizbeth's sidekick with Nibbler. Heloise is popular to the point of appearing in the 2nd spinoff as one of the 3 main characters with Bender and Skipper. She also got a big role in Totally Mobian Spies as well sharing the spotlight with Bender, Skipper and Lizbeth.
The Joker while a vicious and annoying villain he returned for two more stories as a main villain and became fleshed out as Bender's archenemy as their interactions were memorable and people loved watching him gets his ass kicked by Bender.
Hunson Abadeer was a suprise villain as the true villain of The V Team Island Adventure. His cleverness, power and unique abilities made him unique among many villains. So he returned as the main villain in The Great Time Travel Adventure and managed to be the first villain who won control over the multiuniverse for some time. He even came VERY close to winning, he also developed a rival/nemesis to oppose him
Django of the Dead: He alongside Bender is the only character to appear in every story so far aside from Legends of Light and Darkness. He was morally ambigous for the 1st two stories then switched sides to the heroes and stayed there ever since becoming a importnat member of the B Team
Discord: Orginally a villain for The Grand Summer Season Trek, But he went further than supposed to. Discord was revealed as the true villain for The Grand Summer Season Trek and was the man behind everything in Totally Mobian Spies as well as the main villain once again in The Multiversal War of Destiny. He also has replaced Joker as Bender's archenemy, and is an enemy for the whole team and then eventually became the team's friend and ally