Daisy Ridley will return as Rey.
There are certainly some criticisms that can be aimed at Star Wars: The Force Awakens, but it is hard to criticize the new cast members that were introduced. J.J. Abrams did a great job of blending the old with the new and, more importantly, giving fans characters they can really care about moving forward. Daisy Ridley as Rey is specifically someone fans really connected with, and looks to be our main focus of this new trilogy. So Daisy Ridley is definitely coming back in Star Wars: The Last Jedi. And she is going to be a huge part of it.
Possibly more than any other character that is going to be featured in Star Wars: The Last Jedi, Luke Skywalker included, people want to know about Rey. Is she going to turn evil? What is she going to say to Luke? Why was she left on Jakku? And, probably most importantly, who are her parents? Daisy Ridley is surely going to have her plate full in The Last Jedi. She is surely going to be doing some Jedi training and she is probably going to have some pesky things to deal with. Like a very pissed off Kylo Ren, for one. He is known for his temper tantrums and, given how things left off between those two, we can assume he isn't super happy with Rey at the moment. As the Star Wars franchise progresses into the distant future, it is going to require us, as fans, loving new characters and with Rey, we have a character worth loving. So having Daisy Ridley back and learning more about Rey is an excellent reason to look forward to Star Wars: The Last Jedi.
John Boyega is back as Finn.
For having a relatively small resume heading into Star Wars: The Force Awakens, John Boyega was able to make a very big impression as Finn, or FN-2187, if you will. Finn arguably had the most complex emotional arc in Episode VII and he will be an interesting character to watch in Star Wars: The Last Jedi. He bravely held off Kylo Ren until Rey was able to recover and put him out of commission, but Finn was injured badly in the fight. Rey had to leave before he fully recovered, so our two main heroes will be separated, at least for part of the story, in Star Wars: The Last Jedi. It isn't clear what he'll be doing while Rey is presumably off training to be a Jedi, but we can probably guess he'll be on some kind of important mission for the Resistance. Either way, getting to see Finn in action a bit more will be fun and hopefully, we'll get a little more depth to his already interesting character this time around.
Oscar Isaac will continue to fly the skies as pilot Poe Dameron.
If Star Wars: The Force Awakens delivered us an heir apparent to Han Solo, it was most definitely Poe Dameron. He was used sparsely, but he definitely made an impression. Lucky for us, the best pilot in the Resistance is coming back for Star Wars: The Last Jedi and Oscar Isaac will once again be the man playing him. Though, he is probably going to be sporting a new jacket, since he gave his old one to his bff Finn. It isn't quite clear how meaty his role will be in Star Wars: The Last Jedi, but since he assaulted Starkiller base and helped off the First Order at Maz Kanata's palace, not to mention his escape that prevented the First Order from getting the map to Luke, it seems like the bad guys aren't going to be overly thrilled with him. Poe probably earned a great reputation among his peers, but the First Order will probably be out for revenge. No matter how he is used and no matter how much screen time Poe winds up with, having an actor like Oscar Isaac in your movie can only serve to benefit it. So having him around for Star Wars: The Last Jedi is most definitely a good thing.