Here's a link to the Must-Read Omake about her, but I'll summarise here.
Marcelee is a perfectly stable Alpha level human-Avernite xeno hybrid (majority human) psyker created by humans in M24. She was about 8 years old when the Cybernetic Revolt occurred. After the death of her father at the hands of the Men of Iron, she was raised by Sirens due to a pact made between the humans of the age and the Sirens. Over the thousands of years of her life, she has travelled all over Avernus, learning everything she could wherever she went, giving her an immensely varied amount of psychic abilities. She has fought through daemonic incurusions of all four Chaos Gods. She beat Kairos Fateweaver in a game of wits and permanently killed an Exalted Greater Daemon of Slaanesh during the last daemonic incursion. She has Transcendent Control and multiple Paragons under her belt.