Oh I see ! Well I always thought that (が)こわいの meant to be scared of. In this case being "scared of X" though it is a bit odd that she asks the question "Am I scared of X?" When she talks to X there. が is very notion that I find complicated to translate sometimes.
Wa shows the nominatives (sometimes blended with other grammatrical cases like in accusatives/datives/locatives/instrumentals/etc. left-dislocations in the broadest sense, including topicalizations [like "John, I called. = Jon (ni) wa denwa shitoita yo.” and "John, I *called* him. = Jon (ni) wa denwa shita n da yo. [Tegami wo kaita n ja naku te. etc.]) intra-sentence/-sentential "presupposition/presupposed"-ness.
Ga shows the nominatives (sometimes blended with other cases) with intra-sentential "focus"-ness = "info-slot-filler"-ness: "Watashi/Boku/Ore ga (denwa) shi mashi ta (or shita n desu, when that answer must be unexpected to the listener)." = "*I* called John./It is me that called John." in answer to "Who called John? = Dare ga Jon ni denwa shita no (ka/da/desu[-ka]/daroo[-ka]/deshoo[-ka])?"
Ga sometimes show mere non-"presupposition/presupposed"-ness in the wholly-/all-"non-presuppositions" sentences:
A, hito ga taore te ru! (where no info is presupposed)
Kuruma ga kita yo! (ditto)
With the multiple/layered/substrata-ed nominatives-&-other-cases-blendings, you can say things like:
Daitai wa/ga, sen-go wa/ga, senshin-koku wa/ga, toshi-bu wa/ga, josei wa/ga, jumyoo wa/ga nagak-atta.
Thus, you can translate the OP sentence also as "X, do you really mean that it is you (of all people) that are scared?! (Unbelievable! or I could never imagine your/you getting scared!)"
PS:
Wa can be said to be able to express also 取り立て/semantic-contrasts/-reliefs/-highlightings/-"prominences"/-restrictions like:
Jon wa ki (mashi) ta (yo). = (At least) John came (, I'm telling you). (But other guys did not. or As for other guys, I fail to know.)
Jon wa ki wa shi (mashi) ta (yo).
= Jon wa kuru koto wa ki (mashi) ta (yo).
= As far as I know: at least John came; and at least he did the coming, but nothing else.
Here are some easier-to-see explanations of Wa and Ga by some answerers:
"Where is the ..." vs "Where can I find a ..."