can sebastian change his face?
this is just a theory of course, but i think sebastian might not be able to take on a different human form than his current one, and that’s exactly why yana doesn’t show the demon's face in modri’s memories (yes, i believe that demon is him, and revealing his face too early would have immediately dissolved any doubts).
in chapter 14 he tells francis that he was born with that face. 生まれつき means by nature, by birth, native [1].
in bom, instead of changing his face, he uses a mask and makeup, even though it would be more convenient for him to simply change his appearance (sure, ciel told him to act human, but the guy has literally faked his death and resurrection, and the only person besides ciel in front of whom he removed his mask saw his demonic form, so he really could make his job easier if he wanted to) [2, 3, 4]
when he appears to ciel and gets a less-than-positive reaction, he offers to change his form. he changes his clothes, but not his face (as devil’s advocate, this could simply be yana’s laziness or unwillingness to design another face for him). 姿 can be translated as form, shape, but also appearance, dress, guise, picture, image [5].
in chapter 215 he says that his current form was custom-made for ciel. the reading あつらえた is a verb used to describe clothing [6].
all of this could be a coincidence, or it could also imply that sebastian cannot take on a different face. why? my headcanon is that he was once human and later became a demon, but so long ago that he has forgotten many human behaviors. however, nothing in the plot directly supports this, so it remains a very far-fetched assumption.
