PERSONALITY
The first thing to know about Rey, the thing that makes almost everything else about her make sense, is that she has been alone for almost her whole life. Having been abandoned on Jakku at an early age, Rey has had to learn how to survive on her own, and she's managed to do so with varying levels of success. On the one hand she eats, she has a roof over her head, she's alive. On the other hand her only interactions with other people are fighting with other scavengers and occasionally talking to the off-worlders that (mostly) mistakenly end up in the dustbowl. She lives in a fallen Imperial AT-AT and her time is largely spent either studying ships and languages or just sitting alone. Unsurprisingly, her social skills aren't exactly stellar. She often comes off as short or abrasive, or just plain rude, and it takes a while for her to actually warm up to people. She's kind of feral, honestly. But with a heart of gold, of course!
Despite her circumstance though, Rey has managed to maintain an almost childlike fascination with the world. It would be easy for someone in her circumstance to become bitter, cynical and angry, but she hasn't let that happen to her. Her first instinct is still to help someone in need, seen in her first interaction with the droid BB-8. There was no need for her to intervene and possibly risk an altercation with another scavenger to rescue a droid she'd never even seen before, but she does it anyway. While she initially runs after the weird hallucination moment with Luke's lightsaber, seeing Han Solo's death and witnessing the destructive power of the First Order sees her signing up for the Resistance without hesitation and joining the fight. This fight may not have been hers to begin with, but she makes it hers. Rey is also happier than you would expect of someone in her circumstance. Although the events of the movie mean that you don't see it a lot, certainly towards the beginning we see Rey absolutely delight in seeing a new planet covered in forests. She's also absolutely giddy after successfully piloting the Falcon out of a sticky situation on Jakku, and until things get really messed up she can still enjoy the fun moments of an otherwise not-fun trip.
Rey is also ridiculously determined. Like detrimental to herself determined. It's what keeps her on Jakku as long as she stays, what almost sees her running back there despite the opportunity of a lifetime falling into her lap. The single-minded conviction that if she just stayed in Jakku her family would return to her is what kept her trapped on the planet for so long. With the amount of traffic through the outpost Rey almost certainly could have left at any time, she knew herself to be more than adequate as a pilot and had a solid enough understanding of ships to be a mechanic, but she was so determined to find her family in this particular way that it puts blinders on her and keeps her trapped in a pretty terrible position. Without that determination she wouldn't have survived this long, wouldn't have helped BB-8 back to the Resistance, wouldn't have fought Kylo Ren and saved Finn's life, and she damn well wouldn't have gone on some jaunt to find Luke Skywalker, that's for sure.
Ultimately it is Rey's loyalty that defines her. Her loyalty keeps her waiting for a family that has given zero indication of ever returning. That same loyalty sees her fighting for her life to get Finn and BB-8 off of Jakku. It's why her heart breaks watching the man she only met a short while ago die in front of her, why she fights to avenge him. It's why she fights a battle that by all means should have been impossible to win, because it's the only way that Finn had a chance of surviving. ( Also Kylo Ren is kind of a dick, so it's not just sacrificial goodness that fuelled that fight, but still. ) At the end of the day Rey might be difficult to wear down, appearing rough around the edges and abrasive, but it's quick to see that's not even close to all that Rey is. She's got a big heart, she's empathetic and self sacrificing, and if someone can worm their way into her heart, she'll do damn well just about anything to protect them. She's a good ally to have, Rey.
But also sometimes she might beat you up on the advice of a tiny droid. So there's that. It's a mixed bag.