Dr. Aphra, Triple Zero, and Beetee, for striking a great balance between hilarious and terrifying. Became strong enough that Aphra got her own comic series with Triple Zero and Beetee as major characters, and Triple Zero and Beetee got a mini-comic of their own. Aphra also won a Hasbro poll and got her own action figure as a result, even beating out the fan favorite Ahsoka Tano.
Rae Sloane, the sympathetic major antagonist POV who made her first appearance butting heads with Kanan in A New Dawn, then ended up becoming a recurring character in other material including returning as a major antagonist in the Aftermath trilogy, being promoted to Grand Admiral, ending the trilogy as the founder of the First Order.
Sana Starros, Han's ex-girlfriend like Bria Tharen (who was a part of the rebels that stole the Death Star plans) and Salla Zend from Legends, also implied to have been together with Aphra. Fans of the EU were let down when it turned out that she wouldn't be the female lead in the upcoming Han Solo movie.
Star Wars: Darth Vader characters Aiolin Astarte and General Karbin of the Cylo Directive both have some fairly decently crafted personalities and skills that make them stand out amongst the already interesting cast of Cylo Directive cyborgs trying to surpass the force through science.