I don't write a lot of reviews on here, but I wanna make an exception here because the Echoes update gave me words.
Skip, Jana, and Kou are some of the best character mods for this game, they're insanely creative and recontextualize the entire campaign. Skip makes you seek out enemies as a resource, Jana's primary movement mechanic uses the walls that most characters either don't interact with or don't want to interact with because climbing is slow, Kou is probably the most straight-forward of those three but cancelling his moves between each-other gives what would be a pretty basic moveset unique execution.
Echoes doesn't feel as thought-out. The other characters, along with many other characters outside of this mod, have some kind of cost to any level-skipping abilities. Tails' flight slows him down, Knuckles' climbing is slow, Metal needs high speed to keep boost, Skip needs scrap to make shields, Jana and Kou have high skill floors, X Sonic spends rings at an incredibly fast rate to use his best move. The intent for Echoes seems to be to have a reverse snowball effect where making a mistake weakens you and you aren't allowed to make many mistakes, but I think she's far too lenient for this to be interesting, and her particular skillset (combined with the type of game this is) prevents that approach from really working.
She essentially works on her own health system separate from SRB2's normal mechanics of rings and shields, and that's obviously there to make her less forgiving, but then for some reason she doesn't lose abyss when she gets hit during her attacking states. This means that her moves are designed to come at the cost of "fragility" that the moves themselves completely offset. I feel even without that design decision, the lost's health system isn't a good way to balance a character here. The thing is, the lost ultimately has a pretty lenient health system, getting to take 1 hit per room isn't that bad of a deal in isaac, and even if you stretch "per room" to mean "per checkpoint" for this game there isn't exactly a habit of checkpoints being spread far apart even in mod levels. Additionally, Echoes's abilities are simply so good that they would completely negate this downside even if getting hit during void caused you to lose abyss, because srb2 is not a game where it's particularly feasible to get hit when you're flying through the air at high speeds, and the situations where speed would put you into danger are often ones that would kill you regardless of the health system change.
If you were to keep the health system as her gimmick, I would recommend the possibility of changing abyss loss to be more permanent, having him refresh only on level clear, maybe life monitors as well.
Additionally, I actually think her abilities could cost rings. It's strange that rings are totally functionless on her unless she has emeralds, and introducing this cost would at least force her to engage with level design with a bit more thought than just flying over everything for free.
At the very least, I wanna know why getting hurt during her attacks doesn't make her lose abyss, I just don't know what the thought process was there.
Reading that she was inspired by an isaac character left me really confused as well, because you made a character who is followed around by a ghost.... and didn't base her mechanics on Forgotten or Tainted Jacob? Really my very first impressions were just disappointment that Abyss doesn't do much, to the point where you can turn him off and the character continues to function exactly the same. The character is like, finished, so it feels rude to make a comment that amounts to "why isn't this something completely different", so feel free to write off this last paragraph entirely.