Sasahara Keiko, sister of Sasahara Kanji, is non-existent in the anime Genshiken 2.

Essentially, they’ve turned Kuchiki into a surrogate Keiko, giving modified lines that she would have been delivering to Kuchiki instead. I think this is really a shame because Keiko becomes a pretty good character in the second half of the Genshiken manga and that while the overall effect for the sake of the plot is the same, their wildly different character types result in two wildly different deliveries of the same information. Keiko fills a role that Saki can’t, being both normal (if you consider ko-gal to be normal) AND unreliable, and she gets quite a few good lines and even a chapter devoted to her. Probably the best example of Keiko goodness is in Volume 9 of the manga with her interactions with both Ogiue and Ohno.

All this considered, I’m not exactly sure why they’ve done this, seeing as Keiko has appeared as late as the third OVA episode. There are a number of possibilites though. Maybe Keiko’s voice actor, Shimizu Kaori, was unable to fulfill the role. Maybe they figured Keiko wouldn’t be a popular character. Maybe they thought the cast would have gotten too cluttered. Maybe they really like Kuchiki. It’s all speculation though.

At the very least though, I can say that reading the manga and watching the anime are two similar but different experiences. I, of course, recommend doing both.