Taking the shojo magazine illustrators in chronological order, the next person on Gekkan Bijutsu's list is Koji.
According to Tokyo Art Beat, Koji Fukiya (1891-1979) painted in the style of Taisho-era (1912-1926) romanticism before becoming known for his Showa-era (1926-1989) illustrations. He wrote the lyrics for the children's song "Bride Doll" at age 25, and he studied Nihonga (Japanese style painting) before becoming an active member of the Ecole de Paris. His range of expression included Jojoga (self-described as "lyricist painting") which featured heroines "with intelligence and hidden, deep sensuality".
(Shojo Kurabu illustration by Koji Fukiya, from Gekkan Bijutsu used for review purposes only.)
A paper written by Yukiko Yamanaka at the Ritsumeikan University goes on to list Fukiya's apprenticeship under Chikuha Odake (1878-1936), his time spent living in Paris and his involvement in producing "Yumemi Doji" ("夢見童子", "Boy Having a Dream", 1958, Toei Studios), in the relatively new medium (for Japan) of animated movies. The Niigata tourism guide describes his memorial gallery as containing 800 of his paintings as well as original hand-written manuscripts.
(Reijo Kai illustration by Koji Fukiya, from Gekkan Bijutsu used for review purposes only.)
Gekkan Bijutsu states that he debuted in 1912 in Shojo Gaho (Girl's Illustrated) at age 21, and the accompanying illustrations here appeared in Shojo Kurabu (Girl's Club) in 1926, and Reijo Kai in October, 1928. In this sense, while Okamoto was the older illustrator (born in 1888 and appearing in Kin no Fune in 1918), Koji beat him to the punch in being first to see print in a shojo magazine in 1912. But, Koji still wasn't the first illustrator, since the first girl's/women's manga, Myojin, started publication in 1900.
Takei Takeo (1894-1983) was an author as well as an artist and book designer. The Boston Book Company has a wealth of images of Takeo's works. According to the Boston Book page, Takeo published 139 books in his "Kampon" series, which ran from 1935 to 1983. Similar to Paul Klee and Bruno Manari in look and feel, Takei used Kampon to experiment with illustration in all directions he could find. Most of the books were intended for children, with wide-ranging stories and styles (additional information on Kampon is provided by Saunders). He designed the title font for Kodomo no Kuni (Children's Country, 1922-1944) and appeared within its pages from the first issue.
(Cover of Kodomo no Kuni by Takei Takeo, from Gekkan Bijutsu used for review purposes only.)
Along with Kiichi Okamoto and the others, he co-founded the Japan Association of Illustration for Children, and took over the role of critic and image editor of Kodomo no Kuni following Okamoto's death in 1931. He became an editorial adviser to "Kinda Bukku" ("Kinder Book") in 1955. (Taken from the Kodomo no Kuni site.) Later on, when we get to coverage from the Kyoto Manga Museum, we'll see that Takei also drew "caricatures" for the Kodomo Puck manga magazine.
(Kodomo no Kuni illustration by Takei Takeo, from Gekkan Bijutsu used for review purposes only.)
The "Kodomo no Kuni and its artists" page has a number of pictures of works from Okamoto, Takei, Takahisa and Hatsuyama.
I'll take a moment to mention Yumeji Takahisa (1884-1934) here, as Takei was an ardent fan of his, and it's the Yayoi-Yumeji Gallery where the Shojo no Tomo exhibit was held. Born in Oku, Okayama, Yumeji submitted short stories and panel strips to one magazine while still in middle school, which were then published. He became the chief illustrations editor for Shin Shojo (New Girls) magazine in 1916, and was one of the leading illustrators in the "Taisho romanticism" movement that included Koji Fukiya. He provided illustrations for Kodomo no Kuni for its first two years, as well as for other children's magazines. He became ill and died just before his 50th birthday. His artwork was collected by lawyer Takumi Kano and is currently displayed in the Yayoi-Yumeji Gallery near Ueno. Again, from the Kyoto Manga Museum, we'll see that Takahisa also drew "caricatures" for the Kodomo Puck manga magazine. If we include his submission of panel strips to that magazine in account, then Takehisa may be one of the first of this group of illustrators to have published manga as well as his other illustrations.