I've heared of this glitch happening before though it might have been Daisey and Faval

I really wish I knew what caused it.
Asaello might still be saved though if you're pregapred to fiddle with action replay codes. I noticed while playing with the codes that player units can be moved to the enemy side so the other way round should be possible.
It will require some experimentation but basically the side code is 4 offsets behind the bonus stats code* (you know from
the Lord Alvein AR code guide on Gamefaqs). I seem to remember the subsitutes not having fixed offsets and usually writing over the data for generation 1 characters.
If you can attach the save file I might be willing to try and find a code.
*. Say the bonus stats code is 7e2d3bff then the side code would be 7e2d37** where ** should be something like 00 or 01.