Enigmatic, androgynous and maybe even immortal (her exact birth-year is unknown, and flashbacks to 1922 show Subaru appearing no younger than she does in 1928), Subaru was another member of the short-lived European Hoshigumi alongside Ratchet.
She is acknowledged as a genius, and said to have never been defeated in any form of competition or combat.
As such, she tends to come off as a little arrogant and looks at everything in an analytical sense.
Thus she takes an interest in things that she cannot comprehend completely such as Shinjiro's charisma and the concept of jazz music.