I was looking at the manga and then the
Japanese Wikipedia page about the mangaka and found out that she is a woman, TAKENASHI Eri 武梨えり. People apparently used to think she was a man because she draws a self-portrait of herself as a sort of
skinhead.
She has done work for Type-Moon, and has drawings of characters from Fate/Stay Night and Tsukihime in collections called
TAKE MOON.
On her own
home page, she has a couple of images from Kannagi and numerous blog entries. She went recently to Yokohama to see a Macross live event.
She says that Yamakan told her he definitely wanted an actual place to set the anime in, so she told him Sendai, in northern Japan, because she used to live there and some of the images in her mind were from there, but scenes from Tokyo are mixed in in what she draws. He just said: "Okay, let's take some photos there."
She and the staff put up prayer boards at the big shrine in town, the
Shiogamajinja 鹽竈神社, even though Nagi's shrine is much smaller, mainly because that's where she used to go with her family on special occasions. She asked people who want to go and "do a tour of holy sites" not to disturb the residents, lol.
She is the younger sister of mangaka Yuuki Shin'ichi 結城 真一, and apparently was strongly influenced by him. Here is
his web page, where he mentions working with her.
By the way, it certainly is interesting how close the first part of the anime is to the manga. Almost frame for frame. Even that odd slanted image from behind the legs of the newly-embodied Nagi is straight from the manga. I'll bet that by sticking so close to the manga in some places, mangakas will be more relaxed about the things he puts in (like some of the ecchi stuff).