Warning: Spoilers ahead for Rogue One: A Star Wars Story
One of the defining features of Rogue One: A Star Wars Story is the lack of Jedi fighting against the Empire. The only person that we see overtly using the Force in the film is Darth Vader himself, though his scenes do give us a glimpse of just how powerful the Force can be. On the Rebel side, though, the closest we get to a Force user is Chirrut Îmwe (Donnie Yen). One of the mysterious "Guardians of the Whills," Chirrut is a strong believer in the Force and seems to have uncanny abilities despite being completely blind.
Very little is known about Chirrut, since he wasn't present in any of the other films of the series and the only books he's appeared in thus far are directly related to Rogue One itself. Still, there are bits and pieces of his backstory and abilities scattered throughout the film, so here is a quick guide to who Chirrut Îmwe is (and what he can do), in order to better understand this fairly intriguing character from Rogue One.
Chirrut the Guardian
Chirrut is one of the Guardians of the Whills, a religious sect that is tied to the Force. He was one of the sworn defenders of the Temple of the Whills, until the Empire seized the temple to collect the kyber crystals stored within. Ejected from the temple, Chirrut was left to live in the streets and earn a living as a fortune teller of sorts.
Rogue One didn't address exactly who or what the Whills are, though they have a long history within the Star Wars universe; George Lucas originally planned to have the Star Wars films be entries from the "journals of the Whills." At the time, the Whills were to be ancient immortal beings who recorded the history of the universe and were even mentioned in multiple places within the original script. The references to the Whills were eventually changed so that they referenced the Force instead, but at least some of the influence of this early storytelling idea remained... the idea that the story was being recorded by an observer is the origin of the "A long time ago in a galaxy far, far away..." intro to the Star Wars films.
The Whills were also planned for introduction in The Clone Wars and were supposedly the ones who taught Qui-Gon Jinn how to become one with the Force and contact his friends and allies from beyond the grave. Though this was eventually changed within the series, it's clear that there has long been a concept of the Whills as a race that is closely aligned with the Force. To have Chirrut be a Guardian of the Whills suggests that he has likely received some form of specialized training that was created by this ancient Force-sensitive race and that he may have also been tasked in protecting some of their knowledge or customs.
The Blind Warrior
As seen in Rogue One, Chirrut is a formidable warrior despite the fact that he's blind. His abilities aren't necessarily the result of using the Force like Jedi do, however; by all indications, Chirrut's combat abilities are simply a result of his training as a Guardian of the Whills combined with enhanced senses. A point is made to show him listening to the movements of his enemies as Stormtroopers surround him, demonstrating that his combat prowess is much more physically-based than it is founded in any Force abilities.
It was recently revealed that Chirrut's staff contains a sliver of kyber crystal in its lamp, and that he can hear the resonance of both the crystal and the lamp's battery. This plays a part in his combat ability as well, since it allows him to know exactly where the end of his staff is in relation to his enemies. Combining his ability to determine an enemy's location by sound and being able to hear the end of his staff allows him to remain a force to be reckoned with, even though he can't see his surroundings. Being able to focus on multiple sounds at once in combat, as well as maintaining the focus to use ranged weapons such as his lightbow (a traditional weapon of the Guardians of the Whills, and related to the bowcaster) also suggests intense monk-like training.
This isn't to say that the Force doesn't aid him at all, of course. If nothing else, Chirrut seems to have exceptional luck and intuition in combat. His mantra to focus himself directly references the Force, so it's likely that his peace with the Force offers him at least some degree of guidance or protection even though he is not directly manipulating the Force like the Force users we're familiar with do.
Chirrut and Baze
One of the notable things about Chirrut is that he's always seen in the company of the mercenary Baze Malbus (Jiang Wen). The two have long been partners, having trained as Guardians of the Whills together. It's said that Baze was once the most dedicated of the Guardians, though he ended up losing his faith. After giving up his belief in the Force, he chose to use his abilities to become a mercenary assassin instead of trying to protect the Temple of the Whills.
If Baze left Jedha, he didn't stray far; by the time of Rogue One, he was back at Chirrut's side and playing the role of his protector. He was openly skeptical of Chirrut's beliefs, but followed him wherever he went after the destruction of Jedha City. It wasn't until Chirrut's death that he seemed to find his faith, taking up Chirrut's mantra and fighting on in the face of overwhelming odds. He seemed to fare well in battle from this point, though he did sustain some injuries; it took an explosive blast to take him down, suggesting that he had regained some of the same Force "luck" that Chirrut displayed once he reconnected with his faith.
There has been a lot of speculation about the nature of their relationship, and director Gareth Edwards has not denied fan speculations that Baze and Chirrut are more than friends (nor has he confirmed them; he simply acknowledged that fans are free to speculate). It's unlikely that a romance will ever be made overtly canon, however, as the characters are unlikely to have additional appearances in future movies. A book titled Chirrut and Baze is slated for release next year, but it is a middle-school reading book and as such is unlikely to tackle the subject of the characters' sexuality.
Using the Force
For Chirrut, the Force is a constant companion even though he doesn't use it in the way that Jedi or Sith do. In addition to the Force "luck" that he seems to display at times, he is able to sense the flow of the Force around others. While he likely does not have the ability to tell the future through the Force, he can at least intuit what others are planning based on how the Force flows around them. This is most notable when he determines that Andor is intending to kill someone on the mission to Eadu, though it's likely that he also used this ability to tell the fortunes of people in Jedha as well.
It's also possible that the Force is a component in the martial arts style that Chirrut learned as a Guardian of the Whills. Given how strongly the Whills seem to be connected to the Force and Chirrut's ability to sense the flow of the Force around others, it makes sense that he would perceive the flow of the Force during combat. The Force would likely flow darkly around his enemies, highlighting his targets and possibly helping him to find optimal locations to strike during a battle. This would be a much more passive use of the Force than we've seen in other Star Wars movies, but it would fit in with what we know about Chirrut and his order.
The fact that Chirrut can hear the resonance of kyber crystals may also be tied to the Force as well. The crystals are important to both Jedi and Sith, and are most notably used in the construction of lightsabers (and the Death Star), but they also hold a spiritual significance as they are directly attuned to the Force. Hearing this resonance provides him with another means of sensing the Force, and is likely only possible because of how closely he is attuned with the Force through his training.