Finn is a fictional male character who works for Alliance in the Star Wars franchise. He has been portrayed on screen by John Boyega. Finn was conscripted into the First Order as a stormtrooper at an early age, he has no memories of his childhood, He doesn`t even know what his birth name was, all he was given was a code FN-2817 which prompted Poe Dameron to call him Finn. FN-2817 is the name of the cell that Princess Leia was held in during the first film. In his first mission as a stormtrooper, he is sent to Jakku as part of Kylo Ren`s brigade to recover a piece of a map that will lead the First Order to the location of Luke Skywalker whom the First Order want to kill. After witnessing his friends death, he comes to the realisation that he doesn`t want to be a stormtrooper and wants to be an individual. He sets about rescuing Poe Dameron from Kylo`s battle cruiser and escapes from the cruiser in a T.I.E. Fighter that they steal.
He makes a cardinal sin of removing his helmet which stormtroopers are not allowed to do. When Finn does remove, he gets caught and admonished by Captain Phasma but doesn`t punish him too severly.
On Jakku, he is separated from Poe Dameron and makes his way to a settlement on the planet where Rey takes an instinct dislike to Finn because he is wearing Dameron`s jacket and the droid BB8 told Rey that Finn is not to be trusted. He tells people that he`s with the Rebel Alliance which is a lie but Rey falls for it. From that early point, Finn tries to be Reys protector, always trying to grab her hand and lead her away to safety. He leaves Jakku with Rey in the Millenium Falcon which was in a junkyard on the planet. Finn takes on the shooting duties whilst Rey is the pilot. In space, the Millenium Falcon is captured by a battle cruiser which turns out to be captained by Han Solo, a previous owner. When Han agrees to take on Rey and Finn, they`re taken to Maz Kanata`s Cantina on Takodana. It is on this planet that Finn tries his hand at being a Jedi with Luke Skywalkers lightsabre and is initially a little successful but has to give up.
On D`Qar, he finally joins the Rebellion against the First Order. He is reunited with Poe Dameron, the X-Wing Pilot where Finn is still wearing Poe`s jacket from earlier. Finn tells the Rebellion how to destroy the Starkiller Base and elects to go with Han Solo and Chewbacca to the pilot having knowledge of the planets schematics and how to destroy the planet.
On Starkiller Base, Finn reveals that the part of the base that he actually worked on was the sanitation and not any of the weapons or defences that would make the attack easier. Finn revealed that the reason he said the things he did was so that he could come to the base and rescue Rey whom he has fallen for in a big way. A future romance between these two characters looks certain to blossom. On the planet, he faces off against Kylo Ren in a lightsabre battle but he is knocked unconscious so it is Rey who it is has to deal with Kylo`s threat. Chewbacca manages to find Finn, pick him up and take him back to the Rebel Alliance.
When the trailer first came out and everyone saw that Finn was a stormtrooper, there was a furore over it because all stormtroopers were supposed to be clones of Jango Fett and he was the wrong racial mix. However, Finn is a conscripted soldier, someone who was conscripted at a very young age in order to be indoctrinated and trained in order to fight for the First Order.
It was also theorized that he might be the son of Lando Calrissian given that they are the same colour. There is no link between Finn and Lando Calrissian as yet mentioned and probably won`t be. There was also suggestions that Rey was the child of Han Solo and Princess Leia which has also been refuted.
The character that John Boyega took is presumeably the character that was described as Thomas in the casting call notes. They wanted someone new to play Thomas which I presume is Finn. Before Finn, John Boyega had played Moses in the small film Attack the Block about an Anti-Hero who with his victim and fellow hoodlums are trying to survive an alien invasion in a London estate.