His reasons for wanting to overthrow his leader can vary from one incarnation to the next - sometimes he's simply power-hungry, and sometimes he truly believes he would be a better leader for the Decepticons. Also different from one incarnation to the other is just how effective he is at it - and how long he gets away with it before being blasted to smithereens by his leader. It's fortunate that another recurring trait of Starscreams is that they tend to be very bad at staying dead.
He's often nicknamed 'Screamer by the fandom.
Most versions of Starscream turn into fighter jets, and wield small arm-mounted blasters.
Megatron screams his name in rage, Khan-style, at least once per incarnation.
Starscream is the singular weakness of pretty much all Transformers fangirls, including the writer of this entry.
Starscream wishes someday to overthrow Megatron (the leader of the Decepticons) and become the leader of the Decepticons and take over the universe. Though, every time Starscream tries, he never succeeds.
In G1, Starscream is part of a group of Decepticons called "The Seekers". The rest of the Seekers are Thundercracker and Skywarp. Instead of being the leader of the Decepticons, he does lead the rest of the Seekers. The Seekers are Decepticons that all share the same Jet mode, but their color schemes are different.
In the Animated series, Starscream has the ability to clone himself. He has used some of his clones to trick the Autobots by tricking them into capturing the clone Starscreams, and not the actual Starscream. In the same act, one of the clones was a bomb set to blow up when Megatron was there.. so he would die.. but as the story always ends Starscream fails to succeed.
All of his clones represent parts of starscreams personality. They also have different color schemes:
Ramjet - Lier
Slipstream - Impatient intelligience (is what I'm guessing, if not, Starscream's bitch side, lmao.)
Skywarp - Coward
Sunstorm - Suck up (This is why he's Megatron's bitch)
Thundercracker - Ego maniac (Why he still tries to overthrow Megatron after 5 seconds of being ass-kicked by Megatron)
Thrust - Jealousy
That pretty much sums up Starscream's personality. God.
"I live to serve you, lord Megatron," - Transformers movie quote