Like mentioned above, they don't end up together in the anime. The last scene does tease it though, the last shot of both Fuyuki and Momoka in the show features both sitting under a tree, then Fuyuki's fingers touch Momoka's, who smiles and blushes.
In the manga, there have been a couple of scenes showing that Fuyuki does care for her more than what he usually shows especially in vol.20 (calling her Momoka-chan rather than Nishizawa once and covering her with a blanket when they were both sleeping but he noticed she was feeling cold), but no actual development of a relationship.
The anime often skipped around. It adapted even the last chapter of volume 21, but about half of that volume wasn't animated. Volume 15 also oddly only had two chapters adapted (movie 5). I guess you could start from volume 15 and go from there, about half of each volume will be new, although volume 14 had two pretty good chapters that weren't animated for some reason (one of them features the Keroro platoon needing to face all their human friends brainwashed by Kururu, another one is focused on Mois and her mission of destroying planets).
I guess you can skip volume 20, since it's just movie 2, although there are some differences.