Giovanni
Japanese Name: Sakaki
In most the earlier episodes, Giovanni (Sakaki) is simply referred to as "The Boss". He sits at the head of Team Rocket. Sakaki is kind of a mysterious character, never really coming out much or showing his face very often. He has a tendency to run away from a dangerous situation and leave it to his underlings to fix, like the time his laboratory was destroyed by Mewtwo.
Sakaki, as you already know, is the leader of Rocket Dan. The organization was run by his mother before him, and was apparently passed on to him. He is also the gym leader of Tokiwa (Veridian) Gym, and if you didn't know THAT you have a problem. ^_^
His appearance was a bit of a "mystery" in the Anime up until the Tokiwa Gym episode, when his face is finally revealed. Sakaki wears a red suit with a blue shirt and a yellow tie! Note that none of these are bright bold shades. ^_^ He IS pretty tasteful. Still, he's kinda got a squarish head, and he's got short, thinning black hair. I guess that's the best he can do in pulling off that mob boss kind of appearance...
His past is a real mystery. I believe that his motive for catching so many Pokémon is that he would like to own the world. You can see this in Jessie (Musashi) and James (Kojiro)'s motto, as they say "White Hole, We're Waiting for a White Tomorrow (thus the url of my website!)". What does that mean? We (the Rocket Dan) are small now, but we are waiting for our glory. Although we only see four team members in the series (Musashi, Kojiro, Yamato, and Kosaburo) we know that there are many members of his organization.
Sakaki is a very, very good trainer. He has to be, to be a gym leader, ne? His Pokemon are very well trained; when Satoshi reaches Tokiwa Gym Gary and his little fanclub are completely knocked out by Sakaki's 'secret' Pokemon. ^_^ Our Rocket Gang fights Satoshi with his Pokemon, and they nearly defeat Satoshi. But, of course, they lose. They always lose. -.-*
Obviously, Sakaki owns a lot of Pokemon. So far we've seen a few that he owns, but I imagine that's not even a small portion of his collection. His favorite seems to be Persian, as Persian is constantly around Sakaki. Nyasu belongs to Sakaki also, but Sakaki put him with Rocket Dan because Nyase messed up too much. Sakaki tends to favor powerful Pokemon over strong, and he likes ground type.
Ah, a little more about Sakaki is known...from the Pokemon CD drama. Rocket Dan has been involved with the Mewtwo (Myuutsuu) project from the beginning (DUH!)....but...did you know it all started with not Sakaki, but his mother? Sakaki's mother ran Rocket Dan before Sakaki took over, and she was the one who initiated the Mewtwo project (or at least...the concept. She was after Mew (Myuu) long before anyone else, and Sakaki began the Myuutsuu project when he acquired an eyelash from a Myuu). So, Rocket Dan is a family business! ^_^** Sugoi!
Madame Boss
Madame Boss is Giovanni's mother, and she is the one who gave Miyamoto the job of finding Mew. She was also, as you know, the creator of the Mewtwo project. Scary, ne?
Miyamoto
Could it be..it is..Jessie's mother!!! Sadly, Miyamoto was killed in the Andes looking for a Mew. There was an avalanche and she was never heard from again. Whenever she went out on missions, she always carried a picture of her daughter with her.
Growlie
Japanese Name: Flame
Growlie is James' faithful pet Growlithe. He lives with James' parents at the mansion. Even though he's not really a member of Team Rocket, I have to give him credit for saving James from Jessibell (and his parents) more than once.