Hey, guys. Who is the ML? Arsen? Rainer? Prince? Any novel readers around?

It's Arsen ^^

wait isnt rainer her brother

Spoilers ahead
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Arsian is ml. Rainer is her brother. Adopted, yeah, but there's no romance, he really is just cool reliable bro.
Prince is not a real candidate. I'd say he's a problem Camilla needs to solve (the emperor - the main bad guy - for centuries pretty much concider his kids as spare bodies to move his soul into to live forever).

(๑•ㅂ•)و✧