As far as Star Wars goes, Rebels is an underappreciated masterpiece, with The Clone Wars holding a more special place in most fan's hearts and The Mandalorian dominating the mainstream. There are many things that make Rebels so great in its own right, not least of all its characters like Kanan and Hera, two of the best original characters of the new canon thus far.
Season one of the show introduces fans to crucial core characters, such as all of Phoenix Squadron, Agent Kallus, Ahsoka, and others. However, from season 2 onwards, many characters of note debut, some of whom are highlights of the series as a whole.
Gar Saxon
The fall of Mandalore can so easily be seen simply by viewing The Clone Wars and Rebels. Once a world of chaos, violence, and tradition, it is brought under pacifist, neutral rule to become merely a pawn of the Empire.
Gar Saxon represents the latter era, serving as Imperial Viceroy and Governer of Mandalore after the Empire took over the Galaxy, debuting in the show's third season after his "Son of Dathomir" appearances. A traitor of his people, it is so easy to hate Saxon, and like most Mandalorians, he is a solid character and is also important in the story of Sabine, helping fans connect with her more.
Ursa Wren
Speaking of Sabine, one of the most significant aspects of her character is the fact that she is seen as a traitor and has pretty much been shunned by her family, who obeys the Empire to stay alive.
Ursa Wren, Sabine's mother, is not very likable - to begin with. But, she grows as her treatment of Sabine improves. It is great seeing more badass female Mandalorians follow in the footsteps of Satine and Bo-Katan. Even if Ursa, who debuted in season 3, is not as great or fleshed out a character as those two examples, she is still a good addition to Rebels.
Governor Pryce
One of the handful of main Imperial villains in the show, Governor Arihnda Pryce debuted as pretty much the secondary villain of the show in the premiere of season 3.
She is exactly what you would expect from an Imperial Officer villain. Still, she fits incredibly well within the show as a foil for Thrawn, as well as for the rebels she hunts. She has the arrogance, entitlement, and ruthlessness needed to make her a good presence in the series.
Fenn Rau
Debuting in season 2 of the show and sporadically appearing as at first an enemy and then an ally of the rebels, Fenn Rau is another solid Mandalorian character in Star Wars animation.
As the leader of the protectors, Rau sided with the Empire until his group was killed by Gar Saxon and his Imperial Mandalorians. From there, he became an ally to the rebels and then to Clan Wren as they fought against the Empire. He is another character who is aided by the fact he helps along with a member of Phoenix squadron's story, as well as that of Mandalore as a whole.
AP-5
Just like Artoo and Threepio found each other and BB-8 found D-0, the resident droid of Rebels, Chopper, found himself a close droid friend at the end of season 2 of the show.
AP-5 is a great opposite for Chopper, with a clear Artoo/Threepio-like dynamic between the two. AP-5 has some great light humor moments with his dry comments and unliking of pretty much everything that is not up to his standards. He is obviously not as prominent in the show as Chopper, which is good, allowing fans to enjoy him in the show more when he does pop up.
Loth-Wolves
Having first appeared via pictures in The Clone Wars' Mortis arc, the Loth-Wolves ended up becoming a fairly crucial part of Rebels in the final season, especially when it came to Ezra Bridger and Kanan Jarrus' journeys.
They are fascinating creatures, with a special connection to the Force, upholding the light side on Lothal. Their connection with Kanan is extremely interesting, becoming more so when they guide Ezra after Kanan dies. They may not be speaking characters with their own arcs, but they are an intriguing, unique, and powerful aspect of the series' final season.
Inquisitorius
Following the death of the Grand Inquisitor at the end of season 1, the Seventh Sister and Fifth Brother were dispatched to try and deal with the rebels, being the most prominent villains of season 2.
While Vader was positioned as the main villain, the Inquisitorius had more of a consistent role hunting Phoenix Squadron and Ahsoka, resulting in some excellent moments. Neither are as good season-long villains as the Grand Inquisitor, but they certainly are not lacking in quality.
Bendu
Going back to wildly powerful characters connected to the Force while also being shrouded in mystery, the Bendu, the one in the middle, is another captivating aspect of the series.
Debuting in season 3 on Atollon (the location of Chopper Base), the Bendu comes to Kanan and helps guide him in his journey in the Force. He also helps him figure out how to navigate within the Force while blinded. When the Battle of Atollon comes, and the Bendu shows off his power by bringing a storm upon the Empire and rebels before predicting Thrawn's fate and disappearing, he became even more of an entrancing character.
A Handful Of Existing Characters
Rebels is not just filled with great original characters and characters who made their on-screen debut on this series, but also a group of characters who came from either the films or The Clone Wars to make a fantastic impact on the show.
Ahsoka, Vader, and Palpatine all showed themselves in season 1. But, from season 2 onwards, fans meet Maul, who continued his phenomenal legacy by being typically brilliant, as did Obi-Wan in his one-episode appearance. Rex also debuted in the show in season 2 alongside Wolffe and Gregor, three incredibly beloved clones. Other fantastic characters who came after season one include Mon Mothma, Wedge Antilles, and, of course, beloved pirate Hondo Ohnaka.
Grand Admiral Thrawn
In season 4 of The Clone Wars, Maul returned and became one of the show's very best characters and arguably its best overall villain. In season 3 of Rebels, beloved Legends character Mitth'raw'nurouodo, a.k.a Thrawn, did the same in Rebels.
A strategic and tactical genius, a lover of art, a talented fighter, and a ruthless, intimidating figure, Thrawn was the main villain of the show's final two seasons, and a great one at that. He has proven divisive to some, given the quality of his Legends counterpart as well as his differences from his own novel series. However, he is still amazing in the show and is both an awesome character in his own right and a massive help in getting fans invested and rooting for the heroes.