The character Gust is based on the company Gust Incorporated, one of the game's publishers. Her design is based on the Atelier series. Her backstory states that she comes from a country called Salburg, which is the setting of Atelier Marie: The Alchemist of Salburg.
Contributed by DrakeVagabond
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The Martial Processor Unit set was a leftover asset from Hyperdimension Neptunia's original concept, that being a fairly traditional fantasy JRPG game.
Contributed by Kitsune Hawk
The letters in Nisa's name are the initials of the company Nippon Ichi Software America, one of the game's Publishers. Her overall design is inspired by the Prinnies from the Disgaea series, which is was also published by Nippon Ichi Software. Also, the flat chested jokes of her in the game's story are possibly a reference to the same jokes made in the Disgaea series as well.
Contributed by DrakeVagabond
Strangely, the DLC characters 5pb. and RED have no English voice clips outside of battle. If the game's voices are set to English, they will have no spoken dialogue at all in cutscenes, but if it is set to Japanese instead, their cutscenes are fully voiced.
Contributed by Kitsune Hawk
The Four Legendary Heroes are references to various influential people in the games industry:

Yuzusuki - Yu Suzuki (Sega).
Kuterogi - Ken Kutaragi (Sony).
Gheytz - Bill Gates (Microsoft).
Miamoto - Shigeru Miyamoto (Nintendo).
Contributed by DrakeVagabond
There is a glitch in the event "Aristocrat Leader's Hobby" where the last bit of dialogue spoken by Sir Turquoise has the incorrect voice over, and plays a voice over from a different event.
Contributed by DrakeVagabond