Fanservice and that shitty stuff.
You can basically see every character in his underwear, and even if they are just crappy 3DS models, it was more than enough for them to rate that game C.
Curious enough, in my country, we use the "Pegi" system, which basically assign an "age limit" to every game (the age used are 3, 7, 12, 16 and 18), and then put some indication about why the game is rated like that.
And Awakening, Fates and Echoes all have the same Pegi, Pegi 12. But I'm kinda sure one of the indication about why Fates and Awakening are Pegi 12 is "Sex", while Echoes do not have it. I think showing a weapon or extreme violence in a game is enough for the Pegi system to rate the game 12 (because Pokémon, for example, is Pegi 7 from 2013).