Heroes who are believed/mistaken to be the supposed antagonists, but turn out otherwise. They are mainly anti-heroes in nature or simply act like bad guys from the start, but as the fiction where they appeared in progresses further, they eventually revealed themselves as one of the main characters and faces the true/other antagonist or Big Bads, often either worse than themselves or their evil counterparts. They are the heroic counterpart of Faux Affably Evil villains.

These are some notable conditions that met for False Antagonists:

1. Individuals whom start off as jerks but later turn out to be a good guy or, in greatest extent, the messiah. Some may start off as a protagonist villain that may success in achieving their goal, but are ultimately bored with their current life and their life turned out became better once turned to the good side. Examples are Inuyasha from Inuyasha (1996) series and John Silver from Disney's Treasure Planet.
2. In his/her debut, the individual are portrayed as a Big Bad's spy or underling, but ultimately redeem themselves after questioning their actions and morality. The examples for this are Agent Zoil from Paul and Black Widow from The Avengers: Earth's Mightiest Heroes.
3. Anti-Heroes that are supposed to be good guys, but have a darker/nasty side within their personality that made their roles also doubles as antagonist depending on the fiction where they appeared. For instance, they lack of attitude (either being too much arrogant or too much overprotective with those they care about among other faults), and often do not have the heart of a hero. Squidward Tentacles is an example.
4. Individuals with a tragic/painful backstory that explained why they displayed darker attitudes toward protagonists or other good guys upon their first meeting (namely either cold, ruthless, or bitter). They may also be the villain from start due to this upon their debut. Afterwards, their backstory would be revealed by either themselves or their closest allies that made the protagonist realized that he/she is an insecure/tragic individual that need some help. Perfect example would be Vitaly, who displayed bitter behavior toward Alex upon their first meeting, only to revealed by Stefano that Vitaly became like that out of trauma with a circus accident.
5. Characters (in some case unfriendly mentors/guardians) figure that are seemingly hostile toward protagonists, but turned out to simply tests them to see whether their potentials as allies are suitable for him/her to fight the greater threat. Clockwork from Danny Phantom and Satsuki Kiryūin from Kill La Kill are good examples.
6. Some characters are brainwashed/possessed. Protagonists that are dealing with these individuals may thought that they became evil with their own terms, only to later realize that the said character was turned to the dark side as result of possession/brainwashed (reasons vary, but are commonly masterminded by the Big Bads). Once these characters reverts into their former self/snapped from possession/brainwash influence, it is natural that they hold remorse over the damage that they inflicted when still evil, and drive themselves to do their best to undone it. For example, Jeanne from Bayonetta, got possessed by Father Balder.
7. Heroes that are depicted as cruel and evil in a legend or rumor told to the hero but later turn out to be innocent and misunderstood. Their infamous reputation may be due to either being framed by an antagonist/Big Bad for hiding their evil scheme, physical appearance/immense powers that made them feared and believed to be a scourge of the world, etc. Good examples include Arystar Krory III (who the local villagers mistakenly believed to be a vampire due to his teeth-like Innocence), Iron Giant (the citizens mistakenly think it is bad, but it is harmless and wants to learn), and Sirius Black (whose good guy reputation was brutally and cowardly ruined by Wormtail), the latter of whom to this day remains as one of the best examples of a false antagonist. Another example is Discord (who everypony mistakenly believed to be a chaotic, tyrannical villain, but it is later revealed that Discord, while definitely chaotic, has a heart of gold).
8. A character that another hero mistakenly believes is trying to kill them but then it turns out that they're actually on their side and, more often than not, trying to protect them from a real antagonist. Though similar with the 5th case above, these type of false antagonists' portrayal can be ranged from seemingly hostile enforcers/fighters (either a police, detective, or a soldiers) to assassins that hired to after the protagonist to ensure their safety or other similar motives. Hagi and Yuno Gasai are good examples of this.
9. Characters that are confirmed as not truly evil or not a villain at all, but unfortunately, indirectly caused a character to turn to the dark side. How they were responsible for an individual turned to the dark side ranges from accidental humiliation to relationships severing, resulting in a Heel-Face Turn event. In other words, they are considered as good counterparts of Destroyer of Innocence. Example of this would be Alistair Krei, a businessman whom unintentionally pinned Robert Callaghan to the dark side due to ignoring the high possibilities of accidents that may occur on science projects, including the wormhole generator project that ended in disaster and seemingly killed Abigail. Their villainous equivalents would have been Destroyer of Innocence-type villains.
10. Characters whom suffer multiple-personality disorder in the same sense of Dr. Jekyll and Mr. Hyde. After some incident at one point of their life, developed a second, sinister personality that drove them to do villainous actions. While not entirely evil, these false antagonists still a potential threat for the protagonists, especially when their evil personality came out. Examples for this would be Curtis Connors from the Spider-Man cartoon and film series, who tries to regrow his missing arm and he inevitably turns into a human-lizard hybrid that forces Spider-Man to cure him few times, eventually developed The Lizard as his secondary personality, Jackie Hyde who drinks a potion called Formula 55 which is a drug, she transforms into an exact copy of a model from a magazine, she names her "sister" Jacqueline as her alter-ego in Jacqueline Hyde and Mike from the Total Drama Series, who has a multiple personality disorder, and he has Mal as his evil personality.
11. Characters whom trying to keep evil at bay for Necessary Evil some reason that render them bitter/hostile toward the heroes upon first meeting. Reasons are vary, ranging from preventing the malicious side of their loved ones from came out to guarding a place/seal that contain the dangerous forces. Examples would be Ardeth Bay, the Medjai leader that hostile toward O'Connells during the Hamunaptra excavation due to guarding Imhotep's corpse that had he revived, would bring plagues to the land of the living due to curse that he received prior to his mortal death.
12. A character who mistakenly believes to have turned to the dark side for their own selfish reasons, but it is eventually revealed that they did it out of necessity. Itachi Uchiha and Severus Snape are perfect examples of this.
13. Neutral characters that originally not wanted to involved with troubles from any sides, but an action from one of the sides, either it's good or evil, drove them involved anyway. Examples would be Alpha and her mate from Jurassic Park III whom got angry on protagonists as one of their companions stole their egg.