It's so frustrating to have read this manga hoping to discover what these mysteries were and see if Riku and Shizuku would end up together and then be left with the last few chapters.
Yes, we find out that Shizuku's whole deal was that she was sick (never find out with what) and that she would most likely die (never find out if she lives or dies). We never really get to see Riku's response to this. Did he even find out that Shizuku was sick?! When he finally meets up with Shizuku she never mentions it. Takt never mentioned it to Riku. As far as the reader knows, Riku never found out that Shizuku was potentially dying - he ends up thinking she just ghosted him which is just so cruel on the author's part that I can't quite put it into words.
Also, didn't Shizuku's mom drop a business card from the hospital where Shizuku was going at Riku's feet when he was chasing after Shizuku at their school? Nothing came of that. Riku never went there nor asked anyone about it.
Yes, we find out that Takt was dating her to keep her from Riku and that Takt loved Riku romantically. We never get to see Riku's response to this because Takt never tells him exactly what he did - neither verbally nor in his letter. As far as the reader knows, Riku never found out that Takt had romantic feelings for him and never found out that Takt purposefully dated Shizuku to keep her and Riku from going out.
And after all of that, Riku seems to still have fond memories of his two oldest friends who he doesn't really know lied to him for years. Given what information he got from Takt it makes sense that he still looks back fondly at him as Riku is completely in the dark. However, to still look back fondly at Shizuku who, as far as Riku knows, confessed her mutual love for him and then just fucked off to who knows where is bizarre.
I can't help feeling awful for Riku. He's got fucked over by Takt and never found out about any of that, he lost previous time with Shizuku but doesn't even realize that as he apparently has no clue about her illness and now he has to deal with Umimi and Yako's antics.