I suppose it should have been expected, given the author, but this volume feels like where the story starts to get weird; piloting mecha to defend perhaps the last colony ship from giant shape-changing aliens that destroyed the solar system (or at least earth) was just too straight-forward. Though I have to say one of the oddities of the book is inherent to the genre: the tonal dissonance between the sci-fi action mixed with touches of body horror monstrosities contrasted with the shonen fan service and awkward love triangle sub-plot that sees greater development in this volume. On a related note, one awkward thing is characters who suddenly shift roles, like one leg of that triangle who is presented as a very forward younger woman, maybe a bit ditzy or at least overly focused on romance who is suddenly made an assistant commander, though she never seems to fit. And then a character who was mostly a hot-headed young rival pilot becomes a sinister researcher and conspirator, though it is unclear if that is actually him or if he had his mind taken over by an engineered parasite created by an earlier evil scientist.
The volume also provides a flashback to some of the history of the Sidonia and explaining the origin of the weapons used to destroy the alien enemies. Nice to have some background, but the scene transitions are a bit jarring. Also, we learn that a mere century before the start of the story, the population of the ship was decimated and had only 1/1,000th the current number of people. This is presented as the reason for things like people being able to photosynthesize (though it isn't really a fix: the real difficulty for raising children is taking care of them, not necessarily feeding them; also, there isn't a meaningful light source in interstellar space, though I'll assume they have solar lamps set up) and the batch of identical clones that are major tertiary characters.