Well, I mean… a floating skull seems to me to represent death.

Pretty general human cultural phenomenon of a skull representing death.

And… introspection… is portrayed as… talking to it.

Though I guess if there are this many panels of this kind of a set up… the author is likely using this as an established trope…

‘Talking to your inner self isn’t really that scary, even if it might look like it will be, at first glance.’

‘Challenging or confronting your own inner workings may seem to resemble death, but it actually isn’t.’