tl;dr: A manga focused around an incredible relationship with SM as a foundation.  

One of the core aspects of this manga was BDSM, which was really deep. Here SM is very much a window to the soul that does amazing things. It's still honestly hard for me to view real SM as this deep, but it's not like I know anything about the subject beyond what I learned here anyway, so what do I know. I ended up learning a lot, and there was a lot of variety to it so it never made things feel slow. I'd be interested in more works of fiction that approach the topic in such a manner, but I have a feeling that they'd be pretty different so I don't think I'm particularly going to search. The reason I'm so fond of the implementation here is because how it highlights the two main characters and their relationship worked incredibly well. I like characters that are terrible in a lot of ways but come through when it counts in a believable manner, so I really liked Kaoru. His insecurity and his feeling that anyone he truly loves he loves enough to not want them to be held down by him was something I very much sympathized with. His relationship with his mom was also pretty heartwarming. Nana too I liked, and could greatly sympathize with her feeling the pressure of needing to do well for others. But most importantly, I also really liked how they were together, considering they were very much opposites of each other, and that let them work so well together. The relationship itself and the relationship development was incredible. It was tremendously emotionally charged, with the SM being the core of demonstrating that by being windows into their soul. It was strange in that there was a relationship that slowly built to be stronger and stronger, evolving to be stronger than most romantic relationships, but it never officially became romantic. Once again to be clear, this wasn't the type of story where despite a lot happening there's no change in the relationship or characters are being annoyingly dense, but rather there was A LOT of relationship development and the characters never felt annoying, rather their relationship was just different. It's hard to explain though. It was parallel to being romantic in a sense while also under the surface clearly pushing things along that way. I can't really explain it, so I'll just say that ultimately the bond they had was just really amazing and one I was tremendously invested in. The ending was great in terms of setting up an opposite for Kaoru to show his good points, in that he remained a great guy even though it was so easy for him to turn into a wreck like the villain. It was also great to see his ability come through when it counts be taken to extreme with him coming out looking damn impressive which resulted in him finally getting acknowledged, and ultimately I think it served as a great end for his character arc. Similarly, I think Nana also had a solid ending, in that it showed that she had also grown and was more in tune with her feelings, no longer being dependent on Kaoru, but still remaining loyal to him. I don't think the ending went far enough in regards to their relationship however. I think it was perfect in terms of spirit and just a lot of the small things it did perfectly, but I felt it needed to have just a bit more impact. It clearly insinuates a lot, but I think it needed to be clearer in such a way as to give it more oomph. The art was also incredible, which combined with subject matter that really let it shine made it incredibly impressive.