His design is awesome; it is based on Pit's, but diffrent and greatly inspired, as it looks EXACTLY what anyone would expect a "Dark Pit" to look like.
He is by far one of the best characters to be based on another character, to the point where such character could give Wario a run for his money.
He plays a great role as an antagonist, as he’s the complete opposite of Pit. Pit is a kind-hearted, heroic, talkative, and a bit naive angel, but Dark Pit is the cool, more intelligent, less talkative, and anti-heroic dark angel, refusing to side with either the good or the bad gods but is the perfect rival for Pit.
Has an awesome boss fight, as he flies all over the stage to avoid and attack Pit very cleverly, making Pit think more strategically and making for a unique boss fight.
After Pit saved him, he helped Pit by fighting for him and reviving him, making him grateful.
He becomes good allies with Pit as the game progresses, especially after Pit saved his life, while still having his great anti-heroic personality.
He seems to be somewhat more trusting after Uprising, as revealed in Super Smash Bros. during Palutena’s Guidance, in the Pit Vs. Dark Pit battle, it is revealed he works for Viridi and the Forces of Nature now, and in the Pit Vs. Ice Climbers battle, that he might consider teaming up with Pit again.
He became playable in Super Smash Bros. for 3DS and Wii U and also in Ultimate, being one of the first two "Echo Fighters", along with Lucina.
Sometimes, he uses his signature weapon, the Dark Pit Staff. A good example is in the Super Smash Bros. series where he uses it to shoot down foes with it when he uses his Final Smash.
Just like Pit, he is greatly voiced by his Japanese and English voice actors, Minami Takayama and Antony Del Rio, respectively.
Compared to the much nicer and kinder Pit, Dark Pit can sometimes be mean-spirited at times, like in his reveal trailer in Super Smash Bros. for 3DS and Wii U, where he comes behind Palutena and "crashes her party". Not only is that rude, but it somehow makes him look like a stalker.
Despite being given so much potential in the Smash series, he has an unoriginal moveset, which is with the whole unoriginal "Echo Fighter" process where those fighters have the same moveset as their fighter counterparts. Heck, not only did he steal Pit's new moveset in Smash 4, he even stole Zelda and Sheik's Final Smash in Brawl, which means he's a rip-off of both Pit (SSB4) and Zelda/Sheik (Brawl).
While he is popular, he’s not as popular as most other characters; Similar to Pit and Palutena, Ever since Super Smash Bros. Ultimate released he never made a comeback in the Nintendo games since then.
The concept of Dark Pit's design is based on one of Pit's alternate costumes in Brawl.