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, often either worse than themselves or their evil counterparts. This is the heroic counterpart of Rogue Protagonists.

These are some notable conditions that met for False Antagonists:

1. Individuals who 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.
2. In his/her debut, the individual is portrayed as a Big Bad's spy or underling, but ultimately redeem themselves after questioning their actions and morality.
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 depend 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 don't 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 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.
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.
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 revert into their former self/snapped from possession/brainwash influence, it's natural that they hold remorse over the damage that they inflicted when still evil and drive themselves to do their best to undone it.
7. Heroes that are depicted as cruel and evil in a legend or rumour 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.
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, this type of false antagonists' portrayal can be ranged from seemingly hostile enforcers/fighters (either a police officer, detective, or a soldiers) to assassins that hired to after the protagonist to ensure their safety or other similar motives.
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.
10. Characters who 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.
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 varied, ranging from preventing the malicious side of their loved ones from came out to guarding a place/seal that contain the dangerous forces.
12. A character who mistakenly believes to have turned to the dark side for their own selfish reasons, but it's eventually revealed that they did it out of necessity.
13. Neutral characters that originally not wanted to be involved with troubles from any sides, but an action from one of the sides, either it's good or evil, drove them involved anyway.