Stance mirroring
Stance mirroring is a mechanic in which most or all of a character's animations and moves are mirrored if they turn around, mimicking the sprite mirroring of older fighting games. This mechanic results in the characters' front side being visible more often. When mirrored, all hitboxes are also mirrored - for example, a punch that is animated with a character's right arm, when mirrored, will instead use the character's right arm.
In general, a character must have a symmetrical body shape and not wield a weapon in order to be eligible for stance mirroring, though in Ultimate characters who put away weapons when not in use (e.g. Villager's various tools) are generally not affected by this rule. Minor minor asymmetry such as Pikachu's tail and King K. Rool's eye tic can be acceptable, depending on their exact animations and hurtboxes. Compared to Smash 4, characters with stance mirroring in Ultimate are generally animated in a way that better accommodates the feature.
Stance mirroring also affects which hand characters use to hold, swing, and throw items. Turning around will cause a held item to simply teleport to the other hand (there is no animation for it). A fighter with stance mirroring that is reversed (for example, by Mario's Cape) can be seen in a non-mirrored position until they move around or perform an attack.
As some characters mirror while others do not, it is possible for interactions to differ based on whether the characters are facing each other left-to-right versus right-to-left. For example, a mirroring character's thin attack may hit Marth's extended sword arm from one direction, but not the other, as the different directions may change how far his arm extends into the Z-axis.
Stance mirroring is not present in games before Smash 4, though certain special moves and taunts in Melee and Brawl have two different animations for each direction to accomplish a similar effect.
With the exceptions of Marth, Roy, Young Link, Toon Link, Pit, Lucina, Robin, Dark Pit, Chrom, Simon, Richter, and Sephiroth, as well as Link before Ultimate, all characters without stance mirroring face the screen when facing right.