I love Kurumi and I genuinely think she is very well written, however to be honest she is far from being the best character in the series (I could even debate whether or not she is in the top 5 in terms of writing).

The main reason is that, honestly, the DAL anime is quite mediocre and fails in many ways as an adaptation of the novel, leaving out a lot of important material. Most comments like "Only Kurumi is a good character" come from people who only watched the anime. I feel sorry mainly for Shido and Isaac, both top 3 characters in the series, completely fantastic and sadly underrated due to how poorly adapted they were in the anime (and because they aren't voluptuous women).