i don't think the thorns have anything to do with any external character, at least not so literally
i always took it as:
has a unique love for griffith that isn't
quite fully amatory > develops a fully amatory love for guts > is betrayed, offered up as sacrifice, and raped by griffith > serious emotional trauma that shatters her relationship with and feelings relating to griffith up until that point > femto's
this fucks with the fetus she conceived with guts > gives birth to femto rape demon baby which its actual father immediately tries to kill thinking it's femto's > all the implications of moonlight kid > thorn heart symbolic of casca actively trying to "lock away" her heart from being subjected to even
further pain and trauma
since, think about it
presumably she
knows demon infant was originally sired by guts (assuming she knew she was pregnant with it before the eclipse), and that infant was tainted by femto rape
and served as griffith's new body--meaning that in a somewhat-removed-but-still-very-literal sense, guts killing griffith will also entail guts killing his own offspring
couple this with the seeming conflict of emotional interest between our two favorite contrasting-colored birds, and throw in a bit of inference that she at least subconsciously knows
some of the above regarding the infant, and it seems a perfect recipe to me for a woman who's been through that level of trauma would want to keep what's left of her heart locked up and protected
of course there are a million and one ways to interpret that particular image of the dreamworld, and this is but one
All valid points, the only concern being that her heart had thorns around it, though it could represent the tainted demon/ moonlight child, which could intentionally/ unwanted my implicate ties to Griffith (which explains why a falcon resembling Femto was protecting the heart/ fetus like its own egg, in the dreamworld).
i think the dreamfalcon might've been meant to be a bit more literally-interpreted than that--femto's rape is noted as the major trauma event that shattered her worldview and perception of things, and femto's rape produced the demon infant, so it comes as no surprise the worst trauma and the thing it conceived are tied in her mindscape
although, consider this possibility: perhaps casca subconsciously doesn't
want to wake up and have to confront those traumas, so when a witch and witch-in-training jump into her mind, the part of herself that doesn't want to wake up conjures up the most terrifying image it has in its arsenal to protect the most vital fragment of memory--that terrifying image being femto
i mean we already had penis monsters all over the place, so we
know her mind's already assigning "forms" to the rape trauma, and femto is at the center of all that