Quick Answer: Although Luke Skywalker is the central protagonist of the original Star Wars trilogy, his role is vastly reduced in The Force Awakens. For the majority of the film, Luke remains absent. His whereabouts are unknown even to his close friends. Although he only makes a brief appearance in the film, the mysteriousness enshrouding Luke raises a host of questions and becomes a key motivating factor for the characters we do see. These unanswered questions appear to be the soul of the sequel trilogy. Indeed, it is Luke's absence that gives the sequel trilogy substance.
As one of the central characters in the Star Wars universe, Luke Skywalker (Mark Hamill) is one of the most beloved and popular heroes in the franchise, if not film history. Much of the original trilogy centers on Luke fulfilling his destiny, from his humble beginnings as an orphan farm boy to becoming a famed Jedi. He successfully resists the temptations of the Dark Side from Emperor Palpatine, proving that he is stronger, both emotionally and physically, than his father, Darth Vader. At the end of the original trilogy, everything seems to be running smoothly for the Jedi; the second Death Star has been destroyed and Luke begins to train a new generation of Jedi Knights.
At the onset of the sequel trilogy, however, Luke is no longer the central protagonist, let alone a primary player. For the majority of The Force Awakens (2015), Luke remains absent. His whereabouts are unknown even to his close friends. Although he only makes a brief appearance in the film, the mysteriousness enshrouding Luke raises a host of questions and becomes a key motivating factor for the characters we do see. Indeed, it is Luke's absence that gives the sequel trilogy substance.
Mark Hamill in Return of the Jedi (1983)
In the events leading up to The Force Awakens, Han Solo (Harrison Ford) and Luke’s sister Leia (Carrie Fisher) have a son named Ben (Adam Driver). The Force is strong with Ben, and Luke takes him under his wing as one of his apprentices. However, the mysterious Supreme Leader Snoke (Andy Serkis) tempts Ben to the dark side, slaughters all of Luke’s students and burns his academy to the ground. Luke successfully escapes the slaughter, vanishes and, for the second time, becomes the last known Jedi (whether he flees from fear or guilt will hopefully be explained in Episode VIII). We learn that Luke has ventured to the aquatic planet of Ahch-To in search of the first Jedi Temple and has remained there ever since.
Although rumours initially suggested that The Force Awakens was going to center around Luke’s (formerly Anakin’s) old blue lightsaber, the plot actually focuses on finding Luke. The Resistance (the Rebels after thirty years) and the First Order (descendants of the Galactic Empire) are both after the Jedi Master, but for different reasons. The First Order seeks to destroy Luke in order to totally wipe out the Jedi, whereas the Resistance hopes to find Luke - who may be their last hope - and persuade him to help them. Luke is, even after all these years, a new hope for the Resistance (pun intended). But Luke only appears in the final scene of The Force Awakens when he is discovered by Rey (Daisy Ridley), who presents him with his old Jedi lightsaber. Following this climactic moment, the end credits begin to roll.
Mark Hamill in The Force Awakens (2015)
As we only have a few moments of screentime with Luke, it is our only opportunity to examine his appearance and draw any relevant conclusions. When he finally appears, the Jedi Master has aged significantly. When we last saw him, he was a young man fulfilling his destiny as a Jedi Knight. In The Force Awakens, it's clear that time has caught up with him. Although the Jedi looks aged and weathered, it is possible that he has become more powerful than ever. With an appearance that echoes Obi-Wan Kenobi in A New Hope (1977), Luke now has long, grey hair and a full beard. He may look worse for wear, but he also has the air of an erudite Jedi Master.
Little is explained about his life in the years between Return of the Jedi (1983) and The Force Awakens, but we know that he attempted to rebuild the Jedi Order. While he is the last living Jedi, one could speculate that he may have had assistance from Lor San Tekka (Max Von Sydow) and Maz Kanata (Lupita Nyong’o), some unseen guidance from the ghosts of Obi-Wan, Yoda and Anakin, or even help from Leia, Han Solo and the Resistance. His absence raises many questions, making audiences all the more impatient to see the next two films of the trilogy.
Following the reveal of this new information and his brief appearance in the final scene of The Force Awakens, what could the rest of the trilogy have in store for Luke Skywalker? In Episode VII’s final moments, he encounters Rey, possibly the first person he has seen in decades, and confronts an important piece of his history. His reaction appears to be deeply emotional, whether it's due to the lightsaber, Rey’s plea for help or simply the experience of seeing someone after having been in isolation. Could the meeting between Luke and Rey be a long-awaited reunion? Could he be her father, or at least know who her father is?
The Empire Strikes Back (1980)
The fact that Luke’s brief appearance and the task of finding him are so central to The Force Awakens’ plot points to how important he still is to the galaxy after thirty years. He remains a crucial symbol of hope, not just for the audience but for the internal Star Wars universe, and the Resistance in particular. The cliffhanger ending raises a number of questions: will Luke confront Kylo Ren, his former apprentice and nephew? Has Luke sensed Han Solo’s death? If he has, will he seek revenge or redemption? Did Luke somehow reactivate R2-D2 to show the Resistance his location so they could find him? If so, why now? These unanswered questions appear to be the soul of the sequel trilogy.
Luke’s absence from the film's marketing campaign aroused suspicion about his whereabouts and allegiances. The Force Awakens raises even more questions, but offers few answers. It seems fans will have to wait until the next installment to learn more about the last Jedi.