History WalkingSpoiler / WesternAnimation

* ''WesternAnimation/KipoAndTheAgeOfWonderbeasts'' has Dr. Emilia. To discuss her means delving into the backstories of Scarlemange, Lio, and Song, due to her being an antagonistic presence for all those characters that inspired their motivations. Those being Scarlemange's hatred of humans as a result of being a poorly-treated lab animal under her control, and Lio and Song's need to keep their research and findings secret from her to prevent her from destroying mutes.

** Hiroshi Sato, whose part in the MidSeasonTwist turns his entire characterization on its ear.

* [[SecretIdentity The Blue Spirit]] of ''WesternAnimation/AvatarTheLastAirbender''.

** You can't talk about [[MadGod Discord]] without revealing he goes through a HeelFaceRevolvingDoor before earnestly redeeming himself. Or the fact he was disguised as Grogar in the final season.

** In Book 4, Kez repeatedly mentions about her roommate, Morgan, showing a picture of a man posing outside of a castle. In the ninth episode. we learn that Morgan is ''[[SapientHouse the castle]],'' and that the man is Jeremy, who [[spoiler:left the Train after getting over his mother and sister's deaths]].

* ''WesternAnimation/InfinityTrain'': The big reveals of season one is that "the Conductor" is actually a [[TheUsurper usurper]] named Amelia, motivated by the death of [[TheLostLenore her fiancé, Aldrick]]. It's very hard to talk about either of them without revealing that.
** In season four, Kez repeatedly mentions about her roommate, Morgan, showing a picture of a man posing outside of a castle. In the ninth episode we learn that Morgan is ''[[SapientHouse the castle]],'' and that the man is Jeremy, who [[spoiler:left the Train after getting over his mother and sister's deaths]].

* ''WesternAnimation/InfinityTrain'':
** Alrick's very ''existence'' is tied to [[BigBad the Conductor's]] true motivations.
** The first season's ending changes absolutely ''everything'' about the true nature of RobotBuddy One-One.
** It's impossible to talk about Amelia in the third season, as her reasons for entering that season's story arc is completely tied to the fact that she was the Conductor in the first season.

*** Two characters from the finale, May and June, are also this, given they have their backstories tied to Webby and, adoptive family, and F.O.W.L.

*** Two characters from the finale, May and June, are also this, given they have their backstories tied to Webby and her adoptive family.

* Season 2 of ''WesternAnimation/WanderOverYonder'' introduces KnightOfCerebus Lord Dominator. Good luck getting to the Season 2 premiere without prior awareness of the episode's final reveal that [[SheIsTheKing she's a girl]] [[SamusIsAGirl in a towering suit of masculine-looking armor]].

* Season 2 of ''WesternAnimation/WanderOverYonder'' introduces KnightOfCerebus Lord Dominator. Good luck getting to the Season 2 premiere without finding out that [[SheIsTheKing she's a girl]] [[SamusIsAGirl in a towering suit of masculine-looking armor]].

** During the TimeSkip between his last season 2 appearance and the beginning of season 3 (at least eight months), recurring antagonist Agent Kallus goes through a HeelFaceTurn and becomes [[ReverseMole a Rebel spy]], one of the Fulcrum agents. In the second half of season 3, he gets found out by his boss, and in the season finale [[Recap/StarWarsRebelsS3E19ZeroHour "Zero]] [[Recap/StarWarsRebelsS3E20ZeroHourPartII Hour"]] ends up escaping and formally joining up with the rebels, fulfilling his role as an AntiHero. It's near-impossible to talk about him in season 3, and outright impossible in season 4, without giving this away. Pictures of him from season 4, with his GoodCostumeSwitch, also spoil this.

** You can't talk about [[MadGod Discord]] without revealing he goes through a HeelFaceTurn, [[HeelFaceRevolvingDoor before trying out villainy again]] before earnestly redeeming himself.

* ''WesternAnimation/KipoAndTheAgeOfWonderbeasts'' has Dr. Emilia. To discuss her means delving into the backstories of Scarlemange, Lio, and Song Oak, due to her being an antagonistic presence for all those characters that inspired their motivations; those being Scarlemange's hatred of humans and Lio and Song's need to keep their Mute research and findings secret).

* ''WesternAnimation/KipoAndTheAgeOfWonderbeasts'' has Dr. Emilia. To discuss her means delving into the backstories of Scarlemange, Lio, and Song Oak, due to her being an antagonist presence for all those characters that inspired their motivations.

** Simon Petrikov, aka the Ice King, is an interesting case of this, as the character has been in the show since Day One. However, nothing can really be discussed about him without mentioning the season three and four plot twists that completely turn the character and the show's universe on their heads: he's over a thousand years old, having lived as a antiquarian prior to [[WorldWarIII The Mushroom War]], and flashbacks to this era have him referencing real world locations and pop culture. He was the foster parent of Marceline until her early teens, and the magic crown he wore in order to protect himself and her was what drove him [[WithGreatPowerComesGreatInsanity to utter madness]] and made him unable to remember his life as Simon. When she's a little kid, and he reveals that his crown protects him but makes him crazy. Prior to "Holly Jolly Secrets" and "I Remember You", which revealed some of this information, Ice King was played more as a pure joke villain rather than a sympathetic character, Ooo's status as a post-apocalyptic Earth was just WordOfGod trivia instead of something important to the show's developing MythArc, and Marceline's own past was similarly shrouded in mystery. Talking about Simon Petrikov is necessary to explain an important part of the show's lore and why both Marceline and Ice King's behave the way they do.

** Just even ''mentioning'' that Della Duck is part of the reboot falls under this, as the show's first StoryArc covers her disappearance, while the tie-in comics include stories where she's adventuring with Scrooge and Donald.

** During the TimeSkip between his last season 2 appearance and the beginning of season 3 (at least eight months), recurring antagonist Agent Kallus goes through a HeelFaceTurn and becomes [[ReverseMole a Rebel spy]], one of the Fulcrum agents. In the second half of season 3, he gets found out by his boss, and in the season finale [[Recap/StarWarsRebelsS3E19ZeroHour "Zero]] [[Recap/StarWarsRebelsS3E20ZeroHourPartII Hour"]] ends up escaping and formally joining up with the rebels. It's near-impossible to talk about him in season 3, and outright impossible in season 4, without giving this away. Pictures of him from season 4, with his GoodCostumeSwitch, also spoil this.

* ''WesternAnimation/TheVentureBrothers'' has the Blue Morpho. He's not brought up by name at all until season 6, when the Monarch discovers his lair underneath his condemned family home, but his existence puts ''a lot'' of the show's historical events in a different light. Not only is he the Monarch's father, he was a member of the original Team Venture who Jonas Venture constantly mistreated, up to and including having him salvaged from the plane crash that killed his wife and [[WeCanRebuildHim making him into a helper robot for the Ventures as "Venturion"]]. And if ''that'' wasn't enough, despite Kano putting Venturion out of commission for nearly killing a young Rusty, he was later salvaged ''again'' by Dr. Z, becoming the supervillain and Guild councilman Vendata, who [[AmbiguousSituation may have been]] the one to ultimately have Jonas ThrownOutTheAirlock of Gargantua-1.

* Mayor Jones, Angel Dynamite, Mr E and the Chileses in ''WesternAnimation/ScoobyDooMysteryIncorporated''. Even knowing that most of the tropes around these characters are spoilers probably counts as a spoiler! (Well, except Mr E, who's clearly hiding something from the beginning.) There's also the simple ''existence'' of the GreaterScopeVillain, whom turns things [[GenreShift inside out and upside down]] by their mere presence.

** For most of season 1, Shadow-san simply served as a member of the BigBadDuumvirate who had an explained hatred of Carmen during her time with V.I.L.E. Then the season 1 finale revealed that he was actually GoodAllAlong, and his sour attitude towards Carmen was simply an attempt to keep her from throwing her life away by becoming a full member of V.I.L.E. By season 2, he's on Carmen's team.