In Chapter 1, the faces of the other marker test subjects on the computer in the observation room, are actually members of the UI Development team.
Sylvie's ending features a cameo of The Red Dragon from 3 Count Bout.
During the planning phase of development, the Mexico Team was originally going to be made up of Ryo Sakazaki, Robert Garcia, and Lee Pai Long. Over the course of development, Lee was drop in favor of Yuri, who would finally be replaced with Takuma in the final game.
In Part I: The American Dream - Chapter Three: Playing It Bogart, if the player goes past Rico Muerte's room and goes down the hall where the boarded up doorway is, destroy it and follow where the blood trail leads. There's a dead body with a stake in his back, and a word on the floorboard that said "BUFF" written by him with his own blood. This is a reference to the TV series Buffy the Vampire Slayer.
Temjin from Art of Fighting 2 and Jin Fuha from Art of Fighting 3 make a cameo appearance as co-hosts on the Satella News Network interlude.
Baitang from Real Bout Fatal Fury Special appears in the background of the China stage.
When Kirby absorbs Sora and then uses his Magic, it is immediately clear that Kirby does not get a Keyblade of his own. Instead, Kirby uses an ethereal glowing weapon that resembles a keyblade but is entirely made out of light and sparkles that changes color depending on the spell. While this can be interpreted as being a restriction by Disney over the Keyblade's imagery, Masahiro Sakurai on Twitter chalked it up to being done for lore reasons as he said "...only Keyblade Wielders can use Keyblades..." implying that Kirby isn't "chosen" as barer of one and thus has to use the three spells differently.
The Korean exclusive character, Kim Ung Jae, is named after the then the former president of Viccom, SNK's Korean publishing partner. He is a relative of the company's chairman, Kim Kaphwan.
Kim Jae Hoon and Kim Dong Hwan are named after the then leadership of Viccom, SNK's Korean publishing partner. Jae Hoon was the former president and the son of the acting chairman, Kim Kaphwan. Dong Hwan was the acting president and the chairman's brother.
Kim Kaphwan was named after then chairman of the Korean development studio, Viccom. Chairman Kim was responsible for the Korean imports of SNK arcade cabinets and lent his name, as well as the names of his family members, to ensure SNK's Korean characters had authentic Korean names. Like his game counterpart, he was also a Taekwondo practitioner.
In Part I: The American Dream - Chapter Two: Live From The Crime Scene, if the player throws a grenade into the big gap on the wall at the beginning of the level, the objectives will then say "I had declared a war against rats". The rats ahead will use their deagles to kill the player.
In the tutorial level, there's a 'Tar Cafe sign' that has a quote below it's name says "All your base are...". This is likely a reference to Zero Wing's English translation meme "All your base are belong to us".
Tina's character and fighting style were based on real-life Japanese pro-wrestler Manami Toyota.
Bayman's character and fighting style were based on real-life Russian sambo wrestler and former military officer, Volk Han.
Shinnosuke Kagami was modeled after the characters Hiko Seijuro from Rurouni Kenshin, and Shinobu Sensui from Yu Yu Hakusho. The visual design was inspired by the former, and the overall character was based on the latter.
Both Zanetsu and Shikyoh are based on the character, Udo Jin-e from Rurouni Kenshin. Zanetsu was inspired by the design, while Shikyoh was inspired by the character and personality.
Lee Rekka is based on Jet-Li and his portrayal of real-life Qing Dynasty martial artist, Wong Fei Hung, from the 1991 film Once Upon a Time in China. His real name, Lee Cheng Long, is a reference to one of Jackie Chan's former stage names.