Maker Characters, also known as Mascot Characters, refer to a sub group of characters where character each represent a company or gaming series. The term "Maker/Mascot Characters" is a fan created term to differentiate them from the CPUs, villains and NPC. Despite the CPU being the main characters of the series, the Maker Characters are used more often in combat, due to the fact that CPUs usually join in the later half of the game. It should be noted that despite appearances, all these characters are considered human.
The first game introduces the two most important Maker Characters in the franchise, IF and Compa. Despite the storyline, artwork and advertising focusing on the CPUs, these two are used in the party for the majority of the game. In fact if you don't look at the extra scene for guest characters, buy the DLCs or recruit the CPUs, they can be the only character you would use in the game besides Neptune, although this leads to a Bad Ending.
The reason these two are important as they have been in every game of the main series and the remakes and are the only ones who play a major role in the manga and anime.This is because they represent the two major companies that developed the series, Idea Factory and Compile Hearts. It should also be noticed that they have the most interactions with most of the other characters.
The first Maker introduced is Compa. In this game she is a medical student who happens to stumble upon a unconscious Neptune and join her in her her adventure to recover her memories. During one of their adventures they happen upon IF, who in this game is meeting the pair for the first time. In this game she is a guild agent who worships Leanbox's goddess.
This game also introduced 2 guest characters, a concept lost in the later games, Gust (who represents Gust responsible for the Atelier series) and Nisa (called Nippon Ichi in Japan, who represents Nippon Ichi Software America responsible for Disgaea series among others). These two character are almost completely optional but act as guest character unless you buy the DLC to make them party members.The way to unlock these characters is to watch a number of events where you encounter them on the four lands.
Gust first encounter IF and Compa when they are trying to find medicine to help cure Neptune's cold in Leanbox. She encounters them several times again advising them on item and medicine they buy. This is a nod to the Atelier series.
Nisa first appears when Compa tries to stop a monster's attack and Nisa arrives to saves the day. She continues to encounter the team while acting like a hero until she eventually joins them. Nisa also is a special character as she is the first character to appear in another game series, Disgaea 4.
Next character is 5pb. 5pb is a bit of an unique character in this game. First of all, if you don't buy the DLC, she act as a radio idol that give you reports and thanks from the quest givers after you complete their quests. Other then that she is not encountered in the game. After buy the DLC, you have a number of events, almost all with IF, where you begin to befriend the otherwise shy 5pb. You also learn her name is Lyrica. 5pb represents the music/visual novel company of the same name who is believe to provide the music for most of the game as well as the anime. It should also be note that 5pb is the other only Maker character that has speaking role (although she only has one and 2 lines) in the anime as well as a music concert.
Last but not least is the DLC exclusive character Red. Red has no scenes in game if you don't buy her DLC. Once you do, it leads to a number of events again involving IF that eventually lead to her recruitment. Red represent Red Entertainment, responsible to create a number of dating sims with cute girl and ecchi element., including helping Compile Hearts with the Agarest series. This may be a bit of why Red is portrayed as a bit of a yuri character.
It should be noted that 5pb and Red can be recruited once you have access to all the lands while NISA and Gust take a bit of time and story advancements.
In mk2, all the former Maker character return with the exception of Red, who, up until Re;birth 2, didn't make another appearance . All their personalities and basic background story generally remained the same. However there have been a number of changes, to go with the total recon theme of the game.
IF and Compa are retconned into being childhood friends along with Neptune. Compa is still a nurse in training, while IF is instead of a guild agent, is a member of Planetune's Basilcom.
Gust, Nisa and 5pb all return and are recruited in the game without the need of a DLC, but they are retconned into meeting the rest of the team for the first time, to which both Nisa and Gust comment on. They are each recruited on each landmass, Nisa on Lastation, Gust on Lowee, and 5pb on Leanbox. Nisa still act like a superhero and Gust still acts as an alchemist. However, 5pb, while still an idol, work exclusives for Leanbox.
Cave, based on Cave Co. LTD who make various maniac shoot'em ups, is a specialist of Leanbox's Basilcom, who might have a similar job to IF, and old friend of 5pb. She is very protective of her. She make appearance throughout the first Leanbox visit. After getting the ticket, she tries and make a effort to mix into the other members of the party particularly Nepgear.
Falcom, based on Nippon Falcom responsible for the Ys and Legends of Heroes series, is an adventurer who happens upon the party during their journey, but it remains her only appearance until you get the DLC. After getting it, you learn of her many adventures and that she is also an accomplished writer, to which Blanc is a big fan of. It should be noted that she is one of 2 Falcoms in the series and is likely based on the Ys series.
This game introduces a large number of maker character that would be later become main characters in the PS Vita Remakes. All of the new Maker character have purchased a DLC in order for them to appear and all Maker character have to have purchased a DLC to make them playable.