Questions about skins

I just returned from a long break. I had Eileen in my roster from before I quit. It looks like she got a cool skin at some point but it seems to be unobtainable now. When I quit, skins weren't even a thing.

So I guess I have two questions:

1. How are skins usually obtained? Through the shop? Or events?

2. Do we know if skins ever return after they go away the first time? For instance, do I have a chance of ever getting Eileen's skin?