Tarumi is this series's equivalent to Sayaka from Bloom Into You. You end up loving the character and you want them to get their happiness too, but you know it isn't going to be with the one they want.
Well, the author of the AdaShima novels also wrote Sayaka's spin-off, so the two series are sort of related.
Really good spinoff too, though the first girl that fell in love with Sayaka still breaks my heart, doubly so with her little cameo in the last volume.