Dark Pit is Pit's Echo Fighter, and he is very similar to his counterpart as well. His mobility is completely identical, sharing the same walking speed, dashing speed, air speed, falling speed and three double jumps. Similarly, Dark Pit's standard attacks are all identical to Pit's, sharing the exact same frame data, hitboxes, damage and knockback, with only three attacks in his entire moveset having different attributes (two special attacks and his Final Smash). A Pit player will have no trouble playing Dark Pit or vice versa because of their mirrored movesets, but the slight differences the two possess can make Dark Pit a better or worse choice in a matchup.
The only major differences lie in Dark Pit's special moves. The first is Dark Pit's Silver Bow: compared to Pit's Palutena's Bow, Silver Bow's arrows travel in a straight trajectory and can be only shifted slightly up and down. Their limited movement grant them significantly less utility overall, especially in regards to "sniping" offstage opponents and gimping them. Instead, they possess higher overall damage, increased knockback, and faster speed when uncharged, which make them more rewarding when used as a more conventional projectile. They can also KO offstage opponents if charged up. In essence, Dark Pit's arrows are more effective as a standard projectile or on opponents who are in a disadvantageous state, but offer significantly less utility otherwise.
The second is his side special, Electroshock Arm. Compared to Pit's Upperdash Arm, Electroshock Arm is more effective due to its higher damage, electric hitbox (which makes it safer on shield), and altered knockback; given its horizontal trajectory, it sends at an effective edgeguarding angle, and is a powerful tool for KOing especially at the edges of a stage (where it can do so as early as 80%). Whereas the vertical trajectory of Upperdash Arm allows it to set up juggles and KO consistently anywhere on the stage, and its angle also makes it capable of KOing moderately early on stages with low ceilings, Electroshock Arm instead sees more utility as a surprise KOing and edgeguarding tool; Upperdash Arm is mostly limited to setting up juggles until KO percents instead, rendering it somewhat inferior for KOing compared to Electroshock Arm.
All in all, in line with Dark Pit's Idol description in Uprising, the "flawed clone of Pit" is a more difficult character to use projectile-wise due to his stronger, inaccurate projectile, but brings a different finishing tool in the form of Electroshock Arm, which can grant him opportunities to net KOs in the form of hard punishes. Nevertheless, Dark Pit's differences from Pit are not all that significant (similarly to fellow Echo Fighters Daisy, Richter and Dark Samus), and it is largely up to the player to decide whether to use Pit or Dark Pit during battle, since both are traditional "all-rounders" with a more notable preference towards edgeguarding and recovery.
Much like Pit, Dark Pit's results and representation have not been particularly impressive compared to many other characters
Different from Pit
As in SSB4, Dark Pit is almost completely identical to Pit. Unlike in SSB4, he no longer has a weaker forward tilt or a smaller neutral finisher - because of these changes, he now has a completely identical non-special moveset to his original counterpart, with the only mechanical differences between him and Pit being his neutral special (Silver Bow), side special (Electroshock Arm), and Final Smash (Dark Pit Staff). These mechanical differences, while few, are fairly notable: Silver Bow's arrows are faster and stronger but have reduced control, making them more useful as standard projectiles than for edgeguarding or zoning; Electroshock Arm launches at a lower angle, making it better at the edge as a KOing move than Upperdash Arm, but worse from across the stage; and Dark Pit Staff fires a long beam instead of being a focused attack like Lightning Chariot, making it better for catching multiple opponents scattered around, but the move has less power to compensate.
However, Dark Pit's differences are marginal compared to other Echo Fighters (such as Lucina, Chrom, and Ken). Aside from matchups where Dark Pit's higher power or Pit's better camping ability would prove better, choosing Dark Pit over Pit is almost entirely up to personal preference.
Final Smash: Dark Pit Staff
Dark Pit takes out his staff and fires a powerful horizontal shot, quipping "Goodbye!" or "It's time!" (in Japanese, either "貫け!", Penetrate!/Pierce through! or "一撃で決める...", I'll decide with one blow...) as he does so. Getting caught in the beam deals 1% damage, and the main attack deals heavy knockback along with an amount of extra damage which depends on how many opponents were already hit: 39% for the one closest to Dark Pit, 29% and 21% for the further ones.
In SSB4, the move is functionally a clone of Zelda and Sheik's Light Arrow, down to having the screen freeze and the camera zoom in on each character being hit. In Ultimate, however, both Zelda and Sheik received new final smashes, so the move is now unique to Dark Pit.