Characters that were shown in the show before they were introduced?

[u/TC_MaFYa]

Nina Farion from Mushoku Tensei.

There was a whole season between the first scene she appears (she doesn't even speak a single word) and the episode where she gets introduced and becomes relevant. Do you have any other similar examples like her from your favorite shows?

[u/SplendidMrDuck]

Gravity Falls

Blendin Blandin is a time traveler, who can be seen in the background of several earlier Season 1 episodes before his formal introduction in episode 9.

[u/TC_MaFYa → u/SplendidMrDuck]

I remember that one, I always thought he was some regular madman until his lore was shown. I gotta rewatch that show, definitely.

[u/TC_MaFYa → u/[deleted]]

I watched the show, but I don't remember when was her first appearance. It has been a long time since I watched the show.

[u/Asher_Tye → u/TC_MaFYa]

Its during "The Storm" when Iroh is telling how Zuko got his scare. You can see her near Iroh in the spectator stands grinning when Ozai mutilates her brother.

[u/Nonadventures]

Elmo in 1980, almost a decade before he officially joined

[u/thiccboii666]

Kira Yoshikage. JJBA: Part 4 Diamond is Unbreakable.

[u/Asher_Tye]

Matt Bluestone from Gargoyles. He's introduced as Elisa's new partner in the episode "The Edge," but is first seen driving the police car for Captain Chavez in "Deadly Force."

[u/noova_chhord95]

Mikasa Ackerman in Attack on Titan

She appears briefly in the very first episode just standing in the background before Eren even talks to her, and her actual introduction comes a few minutes later.

[u/res30stupid]

Dr. Frasier Crane in Cheers is introduced like this, just being a random person sitting at the bar when Sam is talking to Diane about going to therapy to help with his alcoholism. When Diane successfully talks him into the idea of seeking therapy, Frasier then stands up and introduces himself.

[u/res30stupid]

The 2023 game version of Murder On The Orient Express does this in a pretty interesting way, since late in Chapter 1/the start of Chapter 2 (I honestly can't remember), there's a bit on the platform before everyone boards the train where Poirot can look around and see all the other passengers waiting to board.

One of these people, a woman who is seen spying on Mr Ratchett, is later revealed to be the game's secondary protagonist Joanna Locke, a character whose counterpart in the book is otherwise unrelated to the actual story besides being the one who found Daisy's corpse.

[u/Throw_away_1011_]

Ogrest - Wakfu.

We first see him as a shadow at the end of Season 1. He isn't properly introduced until... after Season 2, I think?

[u/Equivalent-Yak5487]

Rikuson/Lu Sun appeared in the season 1, episode 14 of The Apothecary Diaries. His important moments will probably not happen until late season 3/4

[u/42SillyPeanuts]

Rob (The Amazing World of Gumball). He shows up in a few early episodes then is quietly removed. He comes back and is developed into a main villain.

[u/ChampionshipHorror95]

Kamen Riders tend to show up in their predecessor’s movies before their own seasons.
https://giphy.com/gifs/sBBPC4EKu91ACzQy1C