I read Tsubasa chapter 107.

So, Fay can use magic with sound because it's different to his normal magic.

A LIKELY STORY. I don't think Kurogane believes it either.

Now, where's the Fai backstory???!!!

Do you want to know my theories about Fay? They're mostly just an amalgamation of other theories that I've heard. I've picked the bits I liked best and put them together.

One thing I really don't subscribe to, is the whole: Ashura tricked Fay into loving him and then used Fay as his puppet for evil deads so Fay stopped him and now Ashura will come back and try to kill Fay like the big scary man that he is, it's a good job Kurogane is there to help pitiful Fay overcome his emotional turmoil.
It sounds too easy, and I don't like to think of Fay being used as a puppet. Now, I haven't read RG Veda, so I don't know his personality, but I really doubt that Ashura is just the big bad.
I heard a good theory from someone on the kimihiro.net boards and I like it, it sounds right to me. It's something along the lines of: Fai has to kill Ashura, but really doesn't want to and so traps him and runs away. I think this could be along the right lines, it sounds tragic enough to be CLAMP and makes Ashura and Fay less 2 dimensional. So for now, I'm going with this one.

As to why Fay won't use his magic; well, that's a difficult one. He didn't lose his magic with his tattoo, we know that much. I think that in some way, if Fay uses his magic, it will either let Ashura know where he is or attract Ashura to him (this is clearly the last thing Fay wants). It may be that the tattoo Fay had was some kind of dampener, so that his magic couldn't be sensed by Ashura. This fits in nicely with the theory that Fay was more willing to use magic in Rekort, where there was plenty of other magic around, so his couldn't be noticed as easily. It also fits when you consider what Fay says in chapter 107, that he can use sound magic, because he doesn't usually use it; therefore Ashura won't be able to recognise it as Fay's magic.

I believe that Fay used his magic in Rekort because things were looking a bit shirty. Mokona could no longer transport them, and they were on the brink of being caught. If they were caught, they would likely be imprisoned somewhere, and somewhere where Mokona couldn't use magic and couldn't transport them. Fay did not want to be trapped in one world for a long time with no means of escape (it would make it easier for Ashura to find him), so he used his magic in order for them to leave Rekort.

One thing I have noticed, but can't really get my head around: Fay says a number of times, that he will help as long as his life isn't in danger. Kurogane also picks up on the fact that Fay won't die when they are fighting outside bar Clover. Could it be that if Fay dies then his spell, and Ashura, are jepordised? I also get the contradictory feeling that Fay doesn't really care about his life, but I think this may be mostly fanon, I don't really remember. He clearly didn't care enough to use magic to save his life in Outo, but this could be explained if Fay was somehow able to sense that Outo wasn't real. The jury's still out on this one, I do quite like the fanon in this case, because I like the idea of no-regard-for-his-safety Fay, but I'll have to wait and see if it turns out to be canon too.

One thing I am certain on is that Fay is shifty, with a capital shift. He may not actually outright lie, but he avoids telling the truth when he can. He also seems to be very good at manipulating people to get what he wants.
I don't want to think he is evil, but I wouldn't be too surprised to find out that it's true.

Hyuuuu! I just don't know what to think.