Finn is a fictional male in the Star Wars film series who was 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 severely.
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 Millennium Falcon which was in a junkyard on the planet. Finn takes on the shooting duties whilst Rey is the pilot. In space, the Millennium 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 lightsaber 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 lightsaber 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.
In both the first two films, he has the opportunity to leave the resistance and become free from everything but something holds him back and decides to stay and fight. In the first film, when Finn at Maz Kanata's cantina, his first opportunity to go with some traders to a far off planet, he is about to get on the space ship that will take him away from everything but decides at the last moment not to. In the first film, it is Rey.
In the second film, after Finn wakes up after having been unconscious from the battle on Starkiller Base, Finn is prevented from doing so by Rose Tico. Rose sees Finn as a hero of the Resistance and someone she greatly admires. When Finn tries to leave the resistance in an escape pod, Rose stuns him and pulls him away. Rose is a technician whose job it is to prevent people using the escape pods.
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.
Finn has an immediate interest in Rey although it is not reciprocated. When Rey is away training with Luke Skywalker, Finn becomes friends with Rose Tico, a mechanic who reveres Finn as a Hero of the Resistance. Finn and Rose are sent to the planet Canto Bight where they are to find an expert code breaker who can disable the First Order's tracking computer. On the planet, they meet DJ who offers to help them all too easily
DJ manages to get them into Supreme Leader Snoke's space craft and to the tracking device. When it comes to disabling the device, the First Order appear and arrest them. General Hux wants to execute them but a suicidal attack on the space ship by Admiral Amilyn Holdo changes the situation and Finn is saved from being killed. Finn faces off against Captain Phasma for what may or may not be the last time. With the help of BB8, both Rose and Finn are able to escape from the space ship and rendezvous with the resistance on the mineral planet Crait.
When the First Order with their new improved AT-AT attack the resistance base, Finn decides to launch a suicide mission to knock out the attack weapon. At the last minute, Rose uses her craft to knock Finns out of the way. In the ensuing moments, Rose is killed but not before she has kissed Finn and revealed that she was in love with him which because of his interest in Rey, Finn didn't see it.
With Rose now dead, Finn is reunited with Rey who is as pleased to see him as he is to see her.
The character that John Boyega took is presumably 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.