"So! I must say, I am impressed you got here. Team Rocket captures Pokémon from around the world. They're important tools for keeping our criminal enterprise going. I am the leader, Giovanni! For your insolence, you will feel a world of pain!"
Giovanni serves as the Head of Team Rocket in the anime, never really shown taking action in field work, but rather sends the grunts and other members for that. Jessie, James, Meowth, Butch and Cassidy along with other members, are constantly striving to please him in their actions, usually without his compliance.
In the early episodes of Pokémon, his face was hidden from view and his voice was distorted. However, this version was later dropped as the series went on to become more comical. Giovanni has only been shown once as the Gym Leader of Viridian Gym, in the episode The Battle of the Badge, facing off against Gary. In this episode he used Mewtwo, which he was controlling with a machine, against Gary's Pokémon, brutally defeating him. By the time Ash challenged the Gym, Giovanni left on account of an emergency, leaving Jessie, James and Meowth in charge. Ever since the destruction of the Gym, he had never returned to it, rather it was rebuilt and had Agatha of the Kanto Elite Four take temporary leadership of it. Giovanni in the anime series might be some sort of businessman, because he is revealed to own Pokémon Island.
He possesses knowledge of the other crime syndicates in the Hoenn, Sinnoh, and Unova regions, which are also regions that he does not have a branch of Team Rocket. Despite his dislike of Jessie, James and Meowth, he allows them to try setting up new branches (as they kept following Ash to capture his Pikachu). Upon Team Galactic's defeat, Giovanni began looking into the working of a mysterious organization in Unova as he wished obtain artifacts to further his plans for global domination without confronting them. Giovanni has not been seen since the Meteorite plan's climax and has allowed Jessie, James and Meowth to do as they please in Unova.
While he is rarely shown, he is implied to be very evil and sadistic and it is hinted that he conducts dangerous experiments with Pokemon and is behind his Team Rocket schemes in Kanto, Johto and Unova. In Mewtwo Returns he shows more sadistic and evil personality than in anime, because he tortures Mewtwo even when Domino advises that he could use Mewtwo's power for himself and before that implies that if Mewtwo doesn't surrender he will use his clones on dangerous experiments which will kill them.
Just like in the game he disbanded Team Rocket. In the Manga, he is not the Gym Leader.
FireRed and LeafGreen Arc
He discovered about Deoxys and captured it. He also battled Red and Mewtwo while trying to locate his son, Silver.
In the games
Giovanni first appeared in the Generation I games as the Leader of Team Rocket. After he and his Team Rocket Grunts were defeated at both the Rocket Hideout and Silph Co., Giovanni escapes to an unknown hideout where the protagonist is unaware of. Eventually, it was revealed that he was the Gym Leader of the Viridian Gym, and the final Gym Leader the player faces. Upon defeating him, Giovanni states that he's giving up on Team Rocket and disappears.
Later on, in the Generation II games, Team Rocket still functions without Giovanni, and believe that he will return. The lost Team Rocket attempt several acts, in order to call their former Leader, though he never makes an appearance.
In the Generation III remakes, Giovanni retains his first role in exactly the same way, with the exception of his updated appearance. Also, instead of passing the protagonist the TM(Fissure), he passes him the TM(Earthquake). At the Sevii Islands, a group of renegade Team Rocket Grunts and Admins attempt to continue their schemes but are defeated by the protagonist.
In Generation IV, another branch of Team Rocket led by Executive Archer attempt to relaunch the team by commiting several acts such as at the Slowpoke Well and the Lake of Rage. After being foiled by the protagonist, the new Team Rocket took over the Goldenrod Radio Tower to contact their missing leader but are once again defeated. Archer then disbands Team Rocket forever, much like his leader three years ago.
It was later revealed during the Celebi Event that Giovanni heard the broadcast from his radio in his hideout in Tohjo Falls. After being defeated, he disappeared.
During the event it was also revealed that Silver was Giovanni's son who was abandoned by his father when he met his defeat at the hands of Red. This act reflects on Giovanni's selfishness, something that the much more ruthless Giovanni in the manga would not do.
In Generation V, Giovanni appears in Black 2 and White 2 in the World Tournament mode wearing a Team Rocket uniform.
Giovanni's Mewtwo was created by Giovanni's scientists on the Cinnabar Island. It first appeared in The Battle of the Badge in Giovanni's Gym battle with Gary Oak. It's so powerful it down Gary's Arcanine and Nidoking with little effort. When Gary tried to find what out Pokémon it was, Mewtwo overloaded his Pokédex. In It's Mr. Mime Time, Mewtwo escapes from Team Rocket Headquarters, destroying it in the process.
Giovanni is believed by some to be the father of Ash Ketchum. This is due to his relationship with Delia Ketchum while they were teenagers.
Giovanni's name may come from the word GEO for the first part of his name. Geo is Greek for Earth and this is because he is a Ground type trainer.
It may also be a pun on how most 20's and 30's American gang members had Itallian names like Alfonso (Al) Capone. Giovanni is an English-Italian name and is a gang leader himself (Team Rocket).
Giovanni's outfit in Black and White seems to be based on a general's uniform.
In Pokémon Yellow, Giovanni replaced Kangaskhan with Persian since Yellow was based on the anime.
He may be inspired by the villain of James Bond series, Ernst Stavro Blofeld. In Bond films and novels, Blofeld leads his own criminal organization, the S.P.E.C.T.R.E., to become rich and reign on the world. As a companion, Blofeld has a cat.
In the Pokémon HeartGold and SoulSilver Celebi event, your rival is found out to be the son of Giovanni.
Giovanni's Generation I sprite has shown Giovanni with something that looks like a moustache, although Giovanni doesn't have a moustache.
The first time Ash Ketchum ever met Giovanni was in IL063, when he challenged Giovanni at the Viridian City gym; the two have never seen each other again. Also on that same episode, Giovanni is seen out of the shadows and is seen for the first time.
Also in the same episode, his identity has been shown since the 15th episode that his identify would not be blown.
Giovanni's Japanese name, Sakaki was the name of the leader of Rocket Reds, a gang which bullied the creator of Pokémon, and the one who killed Satoshi's tadpole, Nyorozo.
He is considered to be the main antagonist of the entire Pokemon franchise.