Here's all the differences I've found
Lucina:
- Sword has no tipper sweetspot or hilt sourspot; hitboxes are consistent all throughout
Dark Pit:
- Silver Bow deals more damage, but has a much more restrictive aim
- Electroshock Arm deals more damage and sends horizontally (and obviously has an electric effect, causing greater hitlag) compared to Upperdash Arm
Daisy:
Sadly, I couldn't find any differences between Peach and Daisy. Even their hurtboxes seem to be identical to each other despite minor animation differences in their walk, dash, and idle stances. I may be mistaken though.
Richter:
- Holy Water can kill Red Pikmin and will not cause explosives to instantly combust due to it being aura-based rather than fire-based
Chrom:
- Sword has no hilt sweetspot or tipper sourspot; hitboxes are consistent all throughout
- Has Ike's up-special
Dark Samus:
- Samus's fire attacks are replaced with electric attacks, causing greater hitlag, preventing explosives from instantly combusting, and allowing the moves to kill Red Pikmin in place of Yellow Pikmin
- Has a higher jump
- Has a generally larger hurtbox due to more exaggerated movements
- Has slightly less range on forward-smash
- Up-smash hits shorter opponents
- Has a faster forward and back roll
Ken:
- Runs and walks faster
- Hadoken deals less damage and does not have the Shakunetsu variant
- Shoryuken is fire-based and hits multiple times
- Tatsumaki Senpukyaku hits multiple times
- Has a different back-throw similar to Pikachu
- Has a different hard jab
- Has a different hard forward-tilt
- Has a different forward-smash
Additional notes:
- While Dark Pit's forward-tilt had less knockback in Smash 4, this is no longer the case in Ultimate and it's now the same as Pit's
- It's a common misconception that Dark Samus is floatier than Samus, however their falls speeds are identical