This is only from my understanding but this is what I got,
MC read the novel in which she got reincarnated in, meaning she knows the real identity of Heilbronner which is the young viscount and our ML.
After realizing she got reincarnated, she wanted to live a peaceful life as a civil servant however she jinxed her own fate by accepting the mission, "Capturing Heilbronner" which was supposed to be taken on by another batch of knights, and since rather than catching Heilbronner, MC wanted to "almost catch him" so that after MC knight mission fails. the mission will then be transferred, however because of this ML got interested in MC.
And yes she knows the phantom thief(Heilbronner's) identity but pretending not to know. Huhu I'm sorry my english is really bad
Im kinda confused w the timeline, i thought all these time she ald unmasked the phantom? But then at ch15 she unmasked him? So like all the chaps before she actually never unmasked him but knew his identity from the novel? So the phantom didnt know that she knows his identity all these time?