Finn first met Rey on the planet Jakku, after he rescued Poe Dameron from the First Order. When it appeared the latter had apparently died in his rescue attempt, their relationship at first got off on the wrong foot, when Rey believed he had to stole BB-8's master's jacket, only for him correct them regarding what actually happened. She assumed he was a member of the Resistance. However, he did not reveal that he was actually a former member of the First Order, as he was ashamed what he had done for them.
Over time, the two become good friends. When Finn finally revealed his true identity as a Stormtrooper to Rey, she was not angry with him for lying to her or about who he really was. When she was captured by Kylo Ren, he went to the Starkiller Base to rescue her, despite the fact that he did not know Rey for very long, when Rey is apparently killed by Ren, he tries to avenge Rey, only to be seriously injured by Ren.
Finn first met Han Solo after he and Rey barely escaped from the First Order's military forces on the planet Jakku, he was amazed to learn that, Han fought with Rebel Alliance in Galactic Civil War, and is Luke Skywalker's old friend, over time, the two become good friends.
When Finn finally revealed his true identity as a Stormtrooper to Han, he was not angry with him for lying to him or about who he really was.
During their mission to rescue Rey and destroy the Starkiller Base, Han was killed by Kylo Ren, enraged by his friend's death, Finn tries to avenge Han's death, only to be seriously injured by Ren.
Finn first met Chewbacca after he and Rey barely escaped from the First Order's military forces on the planet Jakku, During their escape from the Guavian Death Gang and Kanjiklub, Chewbacca was injured, when Finn was attending to his injuries, the latter repeatedly tried to strangle him six times, which Finn became annoyed by, however, Chewbacca tried to strangle him, the seventh time, after he complained about how the Wookiee almost killed him, but Finn ultimately forgave him.
Over time, the two become good friends.
Finn and Ren have been enemies ever since he captured Rey and killed Han Solo. He became enraged when Finn used Anakin Skywalker's lightsaber, when he stated that weapon belongs to him, Finn mocks Ren telling him to come and get it (unaware that he is in fact Anakin's grandson, and the lightsaber does rightfully belong to him), the two engaged in a lightsaber duel (as Finn was trying to avenge Han's death), although he was able to hold his own reasonably well for a relatively short period of time and even managed to lightly wound Ren's right shoulder. However, ultimately Finn was defeated by Ren, partly due to his lack of Force-sensitivity, and mainly because of the latter's superior experience and skills in lightsaber combat, as Ren disarmed him of Anakin's lightsaber and dealt him a severe wound to his back.