The Final Fantasy series has a tradition of each game having a character named Cid. These character have varying roles of importance in the series, but they’re almost always a brilliant inventor, scientist, or mechanic. There’s been quite a few versions of the character over the years, so note that not every single instance has been included, just the main games and prominent spinoffs. Also be aware that there may be minor spoilers for some Final Fantasy games.
Cid actually didn’t appear at all in the original NES release of Final Fantasy, but subsequent versions of the game added in a reference to the character so he’d be featured in every mainline game. Cid is mentioned briefly as the ancient Lufenian creator of the airship you use.
Final Fantasy II was the first game that the character Cid appeared in. He offers a taxi service with his airship to Firion and the party, but later on gives the party his airship as he dies.
In Final Fantasy IV Cid is a friend and mentor to two of the main characters, Cecil and Rosa. He’s also the head of Baron’s engineer forces, and develops the airships the kingdom uses. As he develops misgivings he rebels against Baron, and is later saved by Cecil and the party. He joins as a guest party member a couple times.
Cid is actually featured in Final Fantasy V with his grandson Mid, and the two are engineers from Karnak. They created a machine that can harness the power of the crystals, specifically used to power an airship for Queen Karnak. The party interacts with the pair at multiple points and gains the airship from them.
Final Fantasy VII’s Cid has the biggest role yet, as he’s one of the main party members in the game. Cloud and company first encounter him in Rocket Town, and he designed the Tiny Bronco and Highwind airship the party uses. He’s a gruff man, although a gifted pilot and mechanic. Cid Highwind returns in both Advent Children and Dirge of Cerberus, and also appears as the Cid for the Kingdom Hearts series.
Cid Kramer is the headmaster of Balamb Garden, where Squall and his other SeeD members study. He provides the main party with plenty of helpful training to face down their enemies, and reveals to Squall that the garden is capable of flight.
Cid appears in Final Fantasy IX as the regent of Lindblum, and essentially an uncle to Princess Garnet. Due to infedelity he had a fight with his wife Hilda, and was turned into the creature known as an Oglop. Later he’s turned into a frog, but returns to his human form by the end of the game and patches things up with his wife. He cares about Garnet deeply, and is the designer of the airship the party gets called the Hilda Garde.
Although Final Fantasy XII’s Cid appeared as an antagonist, Cid Aulstyne claims the spot of being the first Cid to be a main antagonist. Aulstyne is the leader of the militaristic Militesi Empire, and hopes to subjugate every magic crystal in Orience. It’s because of him that Class Zero has to spring to action.
Cidolfus Orlandeau is known as the “Thunder God” and is the commander of the Knights of the Order of the Southern Sky. He’s a longtime friend of the main character Ramza’s father, and an immensely powerful swordsman. He joins the party at some point, and is the strongest character at the time that he joins.
Cid once again returns for Final Fantasy XV, and is the head mechanic of the Hammer Head garage. In his youth he fought side by side with Noctis’ father, King Regis. He and his granddaughter Cindy Aurum, help the party with car repairs and more.
The Final Fantasy series has a tradition of each game having a character named Cid. These characters have varying roles of importance in the series, but they're almost always a brilliant inventor, scientist, or mechanic. There's been quite a few versions of the character over the years, so note that not every single instance has been included, just the main games and prominent spinoffs. Also, be aware that there may be minor spoilers for some Final Fantasy games.