The King of Fighters series contains many characters, some of which are from other SNK fighting games, and some of which have appeared in other games. Here's a list of characters that have appeared in a fighting capacity since its original incarnation in 1994, including both King of Fighters EX games:

Characters from Fatal Fury

• Andy Bogard (アンディ・ボガード): Younger brother of Terry Bogard. Boyfriend of Mai Shiranui.
• Billy Kane (ビリー・カーン): British bodyguard of Geese Howard. Has a younger sister named Lilly, who dates Joe Higashi.
• Blue Mary Ryan (ブルー・マリー)
• Gato (牙刀)
• Geese Howard (ギース・ハワード)
• Joe Higashi (ジョー・ヒガシ): Muay Thai Champion who befriends Andy Bogard in Japan. Possibly inspired by Adon from Street Fighter series.
• Kim Kaphwan (キム・カッファン): Tae Kwon Do master.
• Li Xiangfei (李 香緋)
• Mai Shiranui (不知火 舞)
• Ryuji Yamazaki (山崎 竜二)
• Terry Bogard (テリー・ボガード): Older brother of Andy Bogard.
• Tizoc (グリフォンマスク): Also known as Griffon Mask. Mexican professional wrestler.
• Wolfgang Krauser (ヴォルフガング・クラウザー)

Characters from Art of Fighting

• Eiji Kisaragi (如月 影二)
• Kasumi Todoh (藤堂 香澄)
• King (キング): May have been inspired by actress Ellen Degeneres.
• Mr. Big (ミスター・ビッグ)
• Robert Garcia (ロバート・ガルシア)
• Ryo Sakazaki (リョウ・サカザキ)
• Takuma Sakazaki (タクマ・サカザキ)
• Yuri Sakazaki (ユリ・サカザキ)

Characters from Ikari Warriors

• Clark Steel (クラーク・スティル)
• Heidern (ハイデルン)
• Ralf Jones (ラルフ・ジョーンズ)

Characters from Psycho Soldier

• Athena Asamiya (麻宮 アテナ)
• Sie Kensou (椎 拳崇)

Characters with appearances outside this series

These are the characters that have appeared in other games, including, but not limited to, the SNK VS. Series.

• Benimaru Nikiado (二階堂 紅丸)
• Chang Koehan (チャン・コーハン)
• Choi Bounge (チョイ・ボンゲ)
• Goenitz (ゲーニッツ)
• Iori Yagami (八神 庵): May have been inspired by musician David Bowie.
• Kyo Kusanagi (草薙 京)
• Leona Heidern (レオナ)
• Rugal Bernstein (ルガール・バーンシュタイン)
• Vice (バイス): Resembles actress Winona Ryder.
• Whip (ウィップ)

Other characters

• Adelheid (アーデルハイド)
• Angel (アンヘル): Female professional wrestler from Mexico. Some of her moves are inspired by The Rock, a wrestler for the WWE.
• Ash Crimson (アッシュ・クリムゾン)
• Bao (包)
• Brian Battler (ブライアン・バトラー)
• Chin Gentsai (鎮 元斎)
• Chizuru Kagura (神楽 ちずる)
• Chris (クリス)
• Duo Lon (デュオロン)
• Goro Daimon (大門 五郎)
• Foxy (フォクシー)
• Heavy D! (ヘヴィ・Ｄ！)
• Hinako Shijou (四条 雛子)
• Igniz (イグニス)
• Jhun Hoon (ジョン・フーン)
• Jun Kagami (華守 純)
• K' (ケイ・ダッシュ)
• K9999 (ケイ・フォーナイン)
• Krizalid (クリザリッド)
• Kula Diamond (クーラ・ダイアモンド)
• Lin (麟)
• Lucky Glauber (ラッキー・グローバー)
• Maki Kagura (神楽 まき)
• Malin (まりん)
• Mature (マチュア)
• Maxima (マキシマ)
• May Lee Jinju (メイ・リー)
• Moe Habana (葉花 萌)
• Miu Kurosaki (黒咲 壬羽)
• Mukai (ムカイ)
• Orochi (オロチ)
• Ramon (ラモン): Professional wrestler from Mexico. Possibly inspired by Rey Mysterio, Jr. from WWE.
• Reiji Oogami (大神 零児)
• Saisyu Kusanagi (草薙 柴舟)
• Seth (セス)
• Shen Woo (シェン・ウー): Possibly inspired by Steve Fox of Tekken.
• Shermie (シェルミー)
• Shingo Yabuki (矢吹 真吾)
• Shinobu Amou (天羽 忍)
• Vanessa (ヴァネッサ)
• Yashiro Nanakase (七枷 社)
• Zero (ゼロ)