Roy's only major differences are his Neutral Special and his fire effects, and Dr. Mario is the equivalent of Dark Pit for Mario. Just Mario with a different Down Special and Down Air, heavier, and very slight differences in how attacks work. Why would it make sense for Dark Pit to be an echo but not Dr. Mario? Also the logic of Roy should not be considered an Echo of Marth makes less sense when property wise Chrom is more similar to Marth than he is to Roy. Only reason he isn't a Marth echo is because Chrom has that similar Neutral Special to Roy.
Roy and Marth have an incredible amount of differences. People just actively refuse to see them because "anime swordsman."
Marth's jab is two graceful, upward swings, while Roy's is a single strike coming up from the side in a reverse grip.
Marth's up tilt is an upward arching slash that hits above and behind, while Roy's is stylistically different and only hits above him. Marth's is an anti-air while Roy's is a combo starter.
Marth's forward tilt is a horizontal slash taken right from his games, while Roy's is a downward chopping motion that hits below platforms.
Marth's side smash is an arching overhead slash that crashes into the ground in front of him, while Roy's is a violent swing that never makes impact with the ground.
Marth's up smash is a pointed thrust upward taken right from his critical hit animation, while Roy's takes the same move but makes it a multihit with a fire effect. Considering this is similar to the differences between Ness and Lucas' versions of PK Fire and PK Thunder, this is a significant difference.
Marth's down air is a parabolic slash beneath his feet while Roy slams his sword straight down.
Shield Breaker, well, breaks shields with a forward lunge, while Flare Blade is meant to be a strong kill move, and can now turn while charging.
Dolphin Slash and Blazer are both very different attacks as well, with Marth having a quick out of shield option and more vertical range, while Roy has more horizontal reach and a shoryuken-like combo move.
Their two variants of Critical Hit are alike in name only. Marth dashes forward with a single strike while Roy stays in place but has a startup hitbox. Both are based on their respective critical hits from their games.
Their dash attacks also have a difference, though it's smaller. Marth swings outward while Roy swings inward. This also affects their hit boxes.
As for mechanical differences, Roy falls faster and runs faster, to better fit his up close and personal playstyle. Their tipper and hilt mechanics also require Marth and Roy to play differently on a fundamental level, making them by definition not echoes.
The moves they share are their down tilt, down smash, all aerials besides down air, Counter, and Dancing Blade/Double-Edged Dance. Half of his moves are shared with Marth, but half are completely different. Roy is not an echo at all.
Meanwhile, Lucina shares all of Marth's moves but with a Balanced Blade mechanic, making her a Marth echo. Chrom, meanwhile, does have some differences from Roy. Aesthetically, his animations are reworked to use a normal grip, but the hitboxes are the same as Roy's. Mechanically, he lacks a fire effect on his attacks, he has a different up special (Soaring Slash is similar, but not identical, to Ike's Aether, as Chrom doesn't throw Falchion like Ike does with Ragnell), and he has his own unique Final Smash with Awakening Aether. Even though he has the second most amount of differences among the echoes, only behind Ken, he still is very much an echo of Roy, not Marth.