Yamazaki became a member of the Hakkesshu because he conveniently shared several snake-like aspects with Orochi.
SNK says that most of Oswald's background (which has not yet been revealed) has already been surmised by the fans.
Athena's one-sided romantic interest in Kyo was purely fanon, until KOF:KYO and the '98 and 2000 dramas strongly hinted that she did harbor some feelings for him. (And then they dropped the ball.)
Creator's Favorite: SNK flat-out admits they love Rugal. They designed him from the ground up to be the most badass boss character there ever was (and that's not paraphrasing by much), and that they make him the final boss in Dream Match games because he's their favorite. This would be annoying, if Rugal didn't viciously beat his way into players' hearts the world over, cementing himself as an icon to the fanbase as much as he is to the folks at SNK. The SNK fandom likely idolizes him more than Capcom fans have idolized the likes of Akuma or M. Bison.
In order to differentiate Kyo from his clones, fans often refer to his NESTS depiction as "Shin Kyo" (as in "True Kyo").
Yaoiori for Iori, thanks to all of his Foe Yay with Kyo. The infamous "Yagami is I'm-a-gay backwards" joke didn't help in this regard; his tendency of throwing out ambiguously gay lines in his special intros with Kyo probably helped even less. This is teased to no end by various official/fan works.
Fathena for Athena's appearance in XII and XIII: Moeness and chubby legs.
Yuri's parody of the Shun Goku Satsu, the Yuri Chou Enbu, is sometimes called the Shin Yuri Satsu.
Kim is sometimes nicknamed as The Beijing King, thanks to the My Way Entertainment fan dub of one of the Fatal Fury OVAs.
Athena had Reiko Fukai in '94, Moe Nagasaki in '95, Tamao Sato in '96, Yukina Kurisu in '97, Haruna Ikezawa (a.k.a. Momoka) post-'98 (Ikezawa also voices Foxy), Robyn Gryphe in the English localization of Maximum Impact, Lily Kong in the English localization of Maximum Impact 2, and Miwa Gardner in the English localization of XII.
Rugal's main voice actor is Toshimitsu Arai, who also voices Heidern. Other VAs are Banjō Ginga (in the '94 Dengeki Bunko drama CD) and Norio Wakamoto (in 2002 only; notable in that this is a rare occasion where the fans preferred the original seiyuu over Wakamoto).
Playing Against Type: Yeah, usually when you speak of Koyasu, you think of a cool-headed, suave character. Him doing a Hero-WorshipperHot-Blooded junior dude like Shingo? Uncalled, but he did it awesomely enough. (And using a tone relatively close to his normal speaking voice!)
Iori isRukusu. In a bit of an inversion; Yasui's role as Iori is arguably his best-known role, so expect the comparisons to come this way, not vice versa.
Katsuhisa Namase voiced Joe in KOF '94. Nobuyuki Hiyama has taken over the role since.
Robert Garcia: Kazuhiro "Kay" Inage was replaced by Mantaro Koichi, who voiced Robert from KOF '96 to 2003. When Robert popped up as a Secret Character in XI (which was based off of his appearance in Mascot FighterNeo Geo Battle Coliseum), Inage returned to voice Robert, and continues to do so in XII and XIII.
Bunsyu Shinoya takes over for Ralf in XII. In English, Scott Casey in the Maximum Impact series, Jeff Manning in XII.
In English, Clark went from Sean Michaels (Maximum Impact) to Eric Kelso (XII).
Kim is voiced by Kazuhiko Nagata in XII, succeeding Satoshi Hashimoto (Hashimoto still voices Terry, though).
Akoya Sogi (who has voiced Mai from the beginning up to XI) has been replaced by Ami Koshimizu as of KOF Sky Stage. Given that Sogi has apparently retired, it's a safe bet to assume that Koshimizu will take over in all subsequent appearances. This could also be the reason behind Mai's absence in XII.
For Kyo, Andrew Roth (his English VA in the Maximum Impact series) was replaced by Peter von Gomm come XII.
Between '94 and '98, Athena's voice changed as much as her clothes, with one for every game. In order: Reiko Fukai, Moe Nagasaki, the aforementioned Tamao Sato, Chris Yukina, and lastly Haruna Ikezawa, which was the one who stuck. It can be presumed that this was because SNK was testing out seiyuu for her.
Haruna Ikezawa voices Foxy in 2001 and 2002 Unlimited Match, in addition to Athena.
Richard Meyer is voiced by Kong Kuwata, who also voices Geese Howard and Duck King.
Eric Kelso voices Takuma, Clark and Benimaru in the English version of XII and XIII.
Monster Maezuka voices Takuma, Ralf, Benimaru, and Choi.
Masahiro Nonaka voices Kyo's clones, Kyo-1 and Kyo-2. Subverted with evil, magical doppelganger Kusanagi, who is instead voiced by Mitsuo Iwata.
However, KOF Sky Stage seems to be using Nonaka instead of Iwata for the voice of Kusanagi, which of course plays this trope straight again.
In XII and XIII, he shares an Englsh VA (Peter von Gomm) with Ash of all people.
King shares her seiyuu (Harumi Ikoma, who also is the announcer in KOF '96, KOF '98, KOF XI, and KOF XII) with Blue Mary. Ikoma also voiced King's brother Jean/Jan in KOF: Kyo.
Greg Irwin voices Joe and Robert in the dub of XII/XIII.
Kazuhiro "Kay" Inage (who voiced Robert in KOF '94 and '95, and returned in XI) also voices Lucky. Mantaro Koichi (who voiced Robert from KOF '96 to 2003) was also the announcer in '97.
Throw It In: Hyena's lines were devised in a collaborative effort between the seiyuu of Hyena, Alba, and Soiree (who consequently became good friends through the experience).
Krauser's Gigatech Cyclone was actually taken from the spinning supers (i.e. the Senpuuken/Hadou Senpuukyaku) utilized by he and Terry in the Fatal Fury 2OVA. His youthful beauty, lack of a mustache and scars, as well as an authentic German-sounding voice, the other hand, were not retained, despite being well-received by fans.
What Could Have Been: Beside the various prototype designs of characters (many of which can be seen as Strikers in 2000), many plot points were shafted or altered:
Orochi Iori was originally planned to be the final boss of '97 (remnants of this manifest as Iori being a Bonus Boss in said game if you beat Orochi with Kyo).
Kyo and Iori were going to be dropped from '99 to put emphasis on K'. They became secret characters instead.
The biggest "what if?" would have to come from 2001. One could only imagine what direction the series would have taken if SNK hadn't gone bankrupt after 2000.
Believe it or not, it was planned for Terry to actually die at the climax of '99note One of the Fatal Fury team endings give off this vibe by having Terry stay behind in the collapsing base, but The Stinger reveals that he survives. Luckily, head management disagreed and brought him back in 2000, avoiding what would have been a shocking Player Punch and the ensuing barrage of Internet Backdraft. However, the "what if" aspect of his death was played up for the remainder of the saga to the point that Blue Mary told Terry how his teammates (or more specifically, her) were worried about him in the team's 2000 ending.
Kim was supposed to debut in the first Maximum Impact under the alias of "Mr. Taekwondo", akin to "Mr. Karate". Instead, Chae Lim took his place, and Kim popped up in the sequel as his good 'ol regular self. Chae Lim herself was chosen over the likes of Jhun Hoon, May Lee, and Buriki One character Seo Yong Song.
Originally, Chang and Choi were going to be paired up with another dangerous criminal to from the "Fugitive Team". Kim was a last-minute addition, thus setting up the whole "rehabilitation" plot.
King's Surprise Rose super move from Art of Fighting 2 was her original DM in '94 but was scrapped for some reason. King's designer then came up with the Illusion Dance DM and it was well received. It was turned back into a DM in 2002: Unlimited Match.
Ralf (and presumably Clark) was supposed to have a moveset revolving around grenades back in his KOF debut as a Shout-Out to Ikari Warriors. This was scrapped due to a possible case of game breaking mechanics. Averted in XII, which gives him explosives in addition to most of his trademark moves.
Lampshaded in the intro dialogue between Kyo and Clark in XIII, wherein Clark explains that the grenades he's wearing are duds, so there's no danger of them going off if Kyo uses his fire on them.
Heidern's moveset and their names were so strange that the designers were going to make him an alien as explanation for his "abilities." However, the lead designer became attached to the character, giving him a tragic backstory instead and keeping him as a human.
Saisyu was supposed to have died during his Early-Bird Cameo in '94. His appearance in the following game was a last-minute addition.
"Chris of the Flames of Destiny" was originally going to have black flames, but the idea was dumped as they would have been too hard to notice in his rather dark stage.
According to the Developers' Blog for XIII, Chang, Malin, Momoko, and Oswald were meant to be in the game. Chang was reduced to a cameo in the India stage, whereas the other three cameo in endings (the Women Fighters Team, Psycho Soldiers Team, and Ash Crimson, respectively).
And Chang (along with Choi) makes the customary Kim Team ending cameo.
Vanessa was supposed to be playable in '99, but didn't make it in time. She makes an Early-Bird Cameo as a Striker in the '99 Evo console release.
The Neo MAX supers in XIII were at first created as extensions to normal supers, but because of testing feedback they were eventually changed into stand alone supers. A few Neo MAX supers in the final build still look like they were supposed to follow up on normal supers, some of them only effective gameplay-wise when used as such.
Adelheid was apparently meant to be playable in XIII, based on some full-body and winpose sketches by Eisuke Ogura.
If unused◊ concept◊ art◊ for '98 and '99 is any indication, it might have been intended for Adel, Rose, and Magaki to join the series as early as the NESTS Chronicles. In particular, one of Rose's prototype designs is shared with that of Whip.
Foxy, who made her debut in 2001, originally wasn't meant to be playable. When it was decided that she'd be added to the roster, it was intended for Diana to join her, although this didn't pan out because of time constraints and Diana remains to this day an NPC.
Raiden's first playable entry in the KOF series (prior to XIII) was going to be in 2003, but the developers eventually decided on using Tizoc instead.
The Japanese version's introduction for '98, "Cipher", as well as the ending theme, "Zero", had a sound clip in it with rather vulgar language in it: "Pay dat shit like he just got yo motherfuckin' check and he's gonna pick it up at the bank." In all foreign releases of the game and OST releases, except for the 15th Anniversary CD, this clip has been edited out and replaced with static.