Basic Trope: A new character appears with the implication that they've always been there, just off-screen.
Straight: Bob's brother Charlie becomes a regular character in the show, despite the fact that Bob has never previously mentioned him.
Exaggerated: Charlie becomes the main character on the show, and it is suggested that he had a heavy influence on the events of previous episodes, despite not being seen or even mentioned during that time.
Downplayed: Bob was actually in the earlier episodes; he was just in a recurring background character. He gets promoted to a minor/secondary character in the new season.
Justified: Bob had good reasons for not mentioning him e.g. he thought that his brother had died in a car accident, and didn't talk about it because it upset him so much.
Charlie, previously a regular character in the show, suddenly disappears, but everyone constantly mentions him as though he was still there.
The characters one episode act like this is Charlie's introduction despite him having been a regular character up until then.
Subverted: It seems like Charlie popped up out of nowhere, but it turns out that he was actually subtly alluded to in previous episodes, and so his appearance isn't so out of place after all.
Double Subverted: Then it turns out that the characters were actually referring to someone else entirely in those situations.
Parodied: Charlie literally appears out of thin air, and Bob casually sparks a conversation with him, to the bewilderment of everyone else.
Zig Zagged: Charlie is in episode #5, disappears for several episodes at a time and no one remembers him, then reappears in episode #11.
Averted: Either Charlie was there from the start, or when he's introduced it's made clear that he's new to the show.
The writers decide to give the main character a younger brother named Charlie to appeal to younger audiences. As for explaining why no one has mentioned him thus far... They Just Didn't Care.
"Let's not even bother explaining how Charlie got here. We gotta leave some things to the audience's imagination!"
Lampshaded: "Hey Alice, you remember my brother Charlie?" "Actually, no I don't, seeing as how you've never mentioned him before"
Invoked: Characters show up for the sole purpose of gags.
Exploited: The main character is a Magnificent Bastard and brings in a hired actor to pretend he is his brother Charlie for whatever reason. Other characters wonder why he never mentioned him before but quickly brush it off.
Defied: A new character is introduced and the main characters are shown meeting him.
Discussed: "Everyone keeps acting as if they've known Charlie their entire lives but this is the first time I've ever seen him."
Conversed: Bob and Alice talk about how annoying it is when shows do this.
Implies: Charlie shows up and, in his first scene, the dialogue is written as such to imply it might not be the first time the cast has seen him but he is still treated as if he were new.
Deconstructed: Charlie ends up becoming The Scrappy, which can even lead to fans refusing to watch the show anymore.
Reconstructed: Charlie gets some Character Development, and the fans decide that his inclusion in the show, if abrupt, wasn't so bad after all.
Played For Laughs: In the middle of the story, Charlie pops up and a main character suddenly says, "Hey, there's my brother Charlie. Everyone remembers him, right?". The cast give a blank stare for a moment, then suddenly start acting as if they've known Charlie for years.
Played For Drama: Charlie appears and everyone acts as if they've known him for years even though it is obvious he is a new character. This is played off as a Mind Screw to the audience.