Good question. A Rocketshipper is best described as a person who truly believes that Jessie and James are meant to be. I am a diehard Rocketshipper. While I am also a neoshipper, one who believes that Cassidy and Butch belong together, and a Gakishipper, one who believes that Ash and Misty belong together (Most 'shippers really have no life! Lol!), I do think that our very own Musashi and Kojiro make the best couple in the series.
First of all, Musashi and Kojiro are always together. True, they are assigned as a team by the Boss, but let's face it: they just can't live without each other. Anytime one of them is seemingly gone, the other one gets very sad. In "Holy Matrimony," when James is thought to have decided to stay with Jessibelle, Jessie walks off saying that the team just won't be the same without James. Of course at that very moment James came in with the Rocket balloon, swooped down to pick up Jessie and pulled her in. He explained that he needed his freedom and wanted to stay with his team. Then there was a handshake (but was it really a handshake?) between the two and they flew off into the sunset leaving poor Meowth behind running after the balloon and yelling. That episode was pretty early in the series, but I think a lot of people were very disappointed when it didn't result in Jessie and James becoming a couple. Back then they were both too stubborn and/or shy to admit anything. Another rocketshippy episode is "The Battle Aboard St Anne. That's where the famous Rocketshippy pic with them hugging sadly and clasping hands came from. Also in that episode when they thought they were going to die they were frantically telling each other...that they always thought the other was very fashionable?! That was probably just a substitute for what they really wanted to say. Still another Rocketshippy episode was the oh-so-original "The Ghost of Maiden's Peak." Jessie was obviously very concerned when James was acting strangely due to being posessed. She even saved his life (Although she made up some excuse that it was only necessary and she wouldn't have done it if she didn't have to.) There are many other episodes where evidence of the Rocketshippers' theory is shown. There are the terror hugs, the happy hugs, the victory hugs...and of course the occasional (possibly accidental) sweet comment by either one of them. Besides the episodes, there are the movies. I have read the synopsis of the fourth movie, in which Jessie gets kidnapped and the only words that James can say are "Musashi left..." (In the Japanese version). He really, really misses her. So as I said, they are the best couple and team in the whole show.
As with any pair, there is, of course, the occasional arguement. Yes I *know* Jessie hits James with various objects. But that just have to do with her temper and mostly due to James'...well as much as I like James...being a wimp. Well maybe not a wimp but at least a whiner at times. I have seen him act bravely before. However these differences do not result in a personality clash, while James does have a slight...yes, feminine side, Jessie can be a bit of a tomboy. So my conclusion is that they fit together just perfectly!
Okay, now for the part that Rocketshippers love to hear! I did "research" and confirm this many times. Jessie and James do finally get together! Don't believe me? The pic below was drawn by Toshiro Ono, the author of the Pokémon manga. It's from one of the manga novels I have called "Surf's Up Pikachu", the last part (part 4) of the manga series. Someone did add the words "Together Forever" to the pic though, that wasn't part of the original picture. I just love their hair! They both look awesome!