So from the looks of it, here's how the Echo Fighters rank in terms of how much they differ from their counterpart.
Virtually the exact same:
- Yeah these are pretty much exactly what it says, they have zero substantial differences to their counterparts and are all cosmetic
- Dark Samus does have two very small functional differences that in the end don't amount to anything (she has electrical properties where Samus would have fire and DS's roll is 2 frames faster than Samus's due to not going morph ball)
Weird substantial differences but...:
- Dark Pit is in a weird spot where he's basically the same as Pit, but he's kinda not either. He only differs greatly with two moves, which are functionally similar to Pit's but also not. Silver Bow is especially different from Pit's Palutena Bow.
- He also has a third smaller difference in the tipper of his side tilt, which is weaker than Pit's. But it's barely worth mentioning.
Visually similar, functionally different:
- Lucina and Chrom have the same gimmick of losing the sweetspot mechanics their counterparts have, encouraging begginners to pick them up over their more methodical versions. So while, on the surface they have the exact same moves, your approach with them has to be completely different.
- Chrom does differ a tad more than Lucina as he has a brand new Up-B that is very similar to Ike's.
Echo In Name Only:
- Surprising no one, Ken is pretty much Echo Fighter in name only as he has a high number of differences from Ryu.
- These differences include: speed, weight, fall speed, different properties for many normals, brand new normals, different properties for his specials, brand new specials, etc etc
- In short, you could consider him as different from Ryu as Ultimate Dr. Mario is from Mario, possibly more.