I think you should study anatomy because all your drawings always have one or all the same mistakes you always repeat.
In this case: The torsos are GIANT and very long, Rin hands are extremelly little and her left arm looks reversed and her hair don't fit her skull. Len body is simply hilarious: Compare the lenght of the ribcage and the pelvis and you will see they simply don't match and are impossible to align. The ears are too low, the eyes are badly aligned vertically and horizontally.
I really like the painterly soft colors you used here. My only critique is that their torsos appear to be too elongated and their hands a bit small in comparison to their heads. Fantastic work on the artbook by the way~
You've done absolutely gorgeuos job with the colours, and they're simply amazing. Everything is bright and glowing, and it creates a really light and airy atomosphere.
But there are still some things that bother me. I'm not sure if it's part of your personal style, but I think there are some oddities in their anatomy. Rin's head looks acceptable, but Len's head looks a bit too tall. Their hands on the other hand (pun not intended) seem guite small, and their bodies seem a bit too long and shapeless.
I just hope I don't sound like an idiot saying that. It's really weird to critisize an artist who is way better than I could ever be.
But this is really gorgeous artwork anyway, no matter how you look at it.