Fighting games are all about creating unique characters that feel distinct from one another in terms of gameplay, style, and personality. When importing a well-known franchise like the DC comics universe into a fighting game, this becomes even more difficult because the personality is already decided, and everything else has to match.
Netherrealms Studios nailed this process with the DLC cast of Injustice 2 by pushing the limits of their game. The focus of each DLC character is different and it's easy to learn what moves are strong and which are weak, while also having fun with these new mechanics.

Atom

Atom posing in Injustice 2
Atom was introduced to Injustice 2 late in the game's life and had very unique ideas put into his design. His physical appearance is somewhat underwhelming with bland colors and an unrecognizable face put to his character model. As a late game DLC character, it's important for them to be widely known by fans and easily recognizable. Many fans felt there were better options.
Atom's gameplay gave him the ability to change his size and zip around the screen delivering potent mixups. His combos and trait were cool and flashy, but the lack of personality didn't make him appealing enough for players to try him out.

Starfire

Starfire Posing in Injustice 2
Starfire's physical design was handled especially well with distinct facial features, costume redesigns, and flashy effects on her moves. Before release, fans couldn't wait to get to practicing with her. Her aerial movement was one of her biggest selling points and it seemed like she would play like a rush down character who could zip across the screen quickly.
Upon release, Starfire didn't meet expectations because she was actually a really powerful zoner. She was one of NRS's most powerful characters released in any game, and it wasn't fun to fight against her. She needed a lot of tweaking before the community was ready to accept her character design.

Darkseid

Darkseid Posing in injustice 2
Darkseid was the first DLC character to join the Injustice 2 cast. He played the role of a boss character very well because of his minion traits and powerful projectiles that seemed unavoidable at first. His physical appearance was very standard for his character but because he is all about battles and destruction, there wasn't much to say about personality.
Having a boss character with a 2 dimensional motivation and personality made for a weaker option than some others NRS have included in the past. Darkseid was a fun zoner with interesting space control so his gameplay was a solid part of his design.

Raiden

Raiden Posing In Injustice 2
There isn't much to be said about Raiden as a character in Injustice 2 because he was essentially the same as the Raiden in Mortal Kombat 10. The physical appearance of the character was great and pleasant to look at for fans of the series, but it was underwhelming to DC fans.
The game already had a Mortal Kombat representative in Sub-Zero, so Raiden felt unnecessary and somewhat lazy. If NRS took the time to change up his gameplay more and made him feel worth playing as an Injustice character, he would probably have been received better. The best part of Raiden's inclusion was probably his Black Lightning skin.

Sub-Zero

Sub-zero posing in injustice 2
Sub-Zero had a lot more going for him in terms of presentation and appearance than the other Mortal Kombat representative in Injustice 2. This is because his costumes were mostly references to the other ninja characters of Mortal Kombat. This let players feel like they could play their favorite classic Mortal Kombat characters in Injustice 2.
The game play for Sub-Zero was exactly what players expected from his Mortal Kombat 10 iteration, and that was a welcome decision. For MK players, they could now hope into a game of Injustice 2 with their friends and already have an idea of what they could do without too much instruction.

Black Manta

Black Manta posing in Injustice 2
Black Manta is one of the less recognizable characters in the DC universe, but he was well received as a DLC character because of how well he was integrated. He had an interesting move set that let him control large portions of the screen, while he also had the ability to move around the screen himself pretty quickly. This mix of styles meant players could express themselves through him effectively, bringing out their own unique game plans.
Black Manta had some flashy headpieces and aesthetic customization options, but at the end of the day, they were a bit underwhelming just because of how unrecognizable the character was to NRS fans. The biggest positive that came from Black Manta was his gameplay, and fans of him didn't mind his appearance so he was an overall successful addition.

Enchantress

Enchantress posing in Injustice 2
After Suicide Squad , Enchantress was at the highest peak of her popularity, and NRS saw the opportunity to capitalize on that by adding her as DLC to Injustice 2. This could've backfired horribly due to the movie's reception at the box office, but instead they came up with something really special. Enchantress has some of the most interesting dialogue interactions with other characters and her costumes are almost unmatched.
On top of physical appearance, her game play is very unique. She works as an anti-zoner of sorts with abilities that reflect projectiles and heal her own health. Adding a character that can heal to a fighting game is always risky business, but she was balanced well and received well by fans after a few fixes.

Redhood

Redhood Posing Injustice 2
Redhood was a set up character that mixed up opponents, so they couldn't tell when and where to block. His gameplay was ironically closest to the Joker's with tricky traps and mix ups based on items. Upon his release, he was seen as one of the stronger characters because his damage was so high, and he played a great keep away game.
Physically, Red Hood was just as edgy as fans hoped and his costumes reflected that part of his personality. Being able to translate a character from a movie or comic book to a fighting game isn't easy, but NRS nailed it with Red Hood.

HellBoy

Hellboy Posing in Injustice 2
Hellboy was a slam dunk of a character for Injustice 2. His comic book look coupled with a great voice actor behind him made him one of the most distinguished characters of the DLC. On top of his physical appearance, Hellboy's gameplay was perfectly in line with his personality and fighting style in his comics and movies.
The heavy swings of his gloved hand were animated with intense attention to detail, that players could almost feel when doing his combos. His super power allowed him to tank hits like the tough guy he is, staring down the opponent while they attack him to no avail. This character was designed with love for Hellboy and it really shows.

TMNT

Leonardo Posing in Injustice 2
The Teenage Mutant Ninja Turtles were the final DLC characters for Injustice 2, and they concluded the game's cycle with a bang. The turtles were essentially 4 characters in 1, all with unique special moves and styles. This isn't something seen in many fighting games, so off the bat, they were a unique addition to the game.
Their gameplay was fun and very indicative of their personalities, so players were able to enjoy the teens in all their glory. The best part of the turtles being in the game is possibly their clever lines they say when put up against the rest of the cast. They were received very well by fans, and still have a special place in some people's hearts as their favorite fighting game characters ever.