Thank you, BH.
I'm not saying that the idea of a copy-cat, or a power negator, isn't an interesting choice. I'm not saying there can't be some awesome dramatic moments stemming from that power set. I'm not saying the idea is inherently bad in any way, nor am I saying the idea can't work in a some mediums.
Not every trope/idea/concept works in every medium. Lengthy flashbacks can be great in a book, but they don't work as well in a movie, and they work even less in a play. Play by post, and a few others, are special because there is no audience -- only other collaborators, all of which are equal. We all have to share the spotlight, and any character that can invalidate another character (or EVERY character) -- even if you add a paragraph about how she had such a rough morning and how her powers have a thousand drawbacks -- still steals that spotlight.
This is kind of PbP Etiquette 101 -- It's the same thing that stops me from bringing in gender-bent Male!Siryn without talking to the guy playing Banshee first -- except a character that has every power effects everybody, present and future. (I don't actually care about the minute differences between Siryn and Banshee -- hopefully my point is clear that two Sound/Voice based Irish dudes would have some overlap.)
Layla, as an NPC, is not beholden to this logic. NPCs are really just plot devices, and I trust the GM isn't just going to turn this into the 'Layla and Friends' show -- or else I wouldn't be in the game.
@Cuchulainn: Sure, even in a PbP, that could be a fun scene -- sounds like a great 1x1 -- but if you're putting it in a group game, you can't just ignore the impact it has on every OTHER character that's also effected by it.