Starscream
Now this is a character. To keep this manageable, we'll be limiting ourselves to the Generation 1 toys and shows.
The first toy started as Jet Robot from the Diaclone line. He was a bit different from the other two seekers, as you had to formally remove some parts of the plane to make him a robot. You didn't have to, but you could.
The figure was pretty basic and reused plenty of times. There was a two pack with Megatron in 1986, but 1989 would see a new figure as part of the Prentender line. Decent, although I still had an issue with the idea of robots in human looking shells. Are they really big humans or really small robots?
Kmart would get the Legends lines, which was just the Pretender figure on a card. Japan, meanwhile, would get a hero set multipack, with Starscream renamed Tornado. 1990 would see the Action Masters line, and yeah, didn't care for this one. While the other figures we talked about at least looked like their show versions, Action Master Starscream looked like he dipped his arms and legs in blue paint.
The file card would set the tone for most portrayals across the franchise. He wants to replace Megatron, relies on treachery and has an overinflated opinion of himself.
He spent most of the comic series frozen but he acted as an underling (not quite second in command though) who always plotted against Megatron. He did manage to kill a bunch of characters in#50 before getting blown up. He got better though and got brought back as a Pretender. They used the shell as an excuse, saying it was healing his body.
He made it to the end of the series, locked in combat with Ratchet. Not a bad way to go, all things considered.
Voiced by Chris Latta (more or less the same as Cobra Commander), this was the perfect portrayal for many fans. Right from the first story arc he was plotting against Megatron. All Megatron had to do was trip and Starscream would declare himself leader.
The show would have some character development though. We learn that before the war, Starscream was a simple scientist and even best friends with Skyfire.
One of the bigger shocks from the movie was actually seeing Starscream being made leader, for about.05 seconds before Galvatron turned him into dust. He'd come back though, as the first Transformer ghost (including a crossover with the Ghostbusters too). The show even gave him a proper ending. Hurtling screaming through space isn't much of an ending but is an ending all the same.
Starscream would set the tone for treacherous seconds in command, which isn't too shabby for a toy line.