Hey all, there's a music theory reason why these melodies work together and have a creepy, wandering, or sad sound. They're both in the key of G minor.
The notes in the G minor scale are:
G, A, Bb, C, D, Eb, F (F#), G
Their melodies follow the following pitch outlines:
Yone - Starts on an upper G and slowly descends. Instrument sounds like an Erhu, common in Chinese orchestras.
High G, Eb, D, C, Bb, A. There is no low G present at the end of his melody but if you listen to it enough times your ear can hear that that's where the melody wants to go next. It's the resolution.
Yasuo - Starts on low D, jumps up to the G and wanders around that area a bit with some pitch bends. Pitch bends are a common playing technique in Asian flute playing. Sounds like a shakuhachi flute.
D, G, A, G, A, Bb, A, G. This melody plays around with pitch bends on the A and Bb note. This is important as the Bb is one of the notes in the scale that makes it minor or sound sad so bending notes in and out of Bb really brings out that lonely sound.
This is just a basic analysis of the melodies. I'm happy to explain in further detail other fine nuances that contribute to the overall effect of the sound such as the lack of F or F# in either melody or the use of tonic/dominant relationships.