Lazycakes, just buy the light novels. The amount of THIS story to side chapters is about 50/50 or 60/40 (70/30 at most) and it will catch you up fairly well... 'sides it will actually support the author.
On a darker note... Knowing how she turns out, purely from the second volume of the light novel (which we have surpassed), this chapter was more depressing to me than it otherwise would have been.
If I didn't have faith that I knew WHY she turns out how she does (and what her intentions are) I would have been forced to drop this with the final (side) chapter of the light novel...
But, I won't provide any spoilers, 'tis up to all of you to read and judge yourself.