Yoshitoshi was to Lain as Sadamoto was/is to Evangelion. He had a lot more to do with the project than just character designs; that's just what he's credited for.
I think the key was Chiaki Konaka, the scriptwriter. I've heard that they were friends even before ABe went to work at Triangle Staff for the SEL Playstation game project*, and some sources suggest they both collaborated on that part of the production as well as the eventual animation.
The concept in its entirity, I think, belongs to Yasuyuki Ueda; however Lain herself is a creation of Yoshitoshi ABe.
*Yes, production of the playstation game came before production of the anime, as stated by Konaka in an interview with HK, the anime was only released first.
Incidentally, Takahiro Kishida was also a character designer for Lain; though not as much for the lead characters as he was for secondary and background ones. In actuality though, his work is more prominent on screen than ABe's was.
