Ikeda Akihisa worked on the volume 9 to 15 of Angel Densetsu.
But no, Norihiro Yagi have a lot of assistants with Angel Densetsu but they have so much difference of age (some have 40 years, other 16-20 (like Ikeda at this time).
In Rosario GuideBook, you have a interview of Norihiro Yagi & Ikeda Akihisa.
About Kokoa, do you know the author put her in the first volume of season 2 because of the anime ?
The studio wanted to use her fastly so he did that ...
Norihiro Yagi wanted a lot more action in Claymore animé.
Ikeda Akihisa thought the anime of Rosario + Vampire was so much more ecchi and pervert than the manga.
For the Guidebook, i spoke about this one :
http://www.manga-news.com/public/ima...ook-tonkam.jpg (For members who don't know what i spoke about).
The colored chapter (with the Crow flute) was scanlated from it.