Rek'Sai is the queen of her Voidborn species (the Xer'Sai) as well as the largest among their number.
Rek'Sai came to Shurima from the Void millennia ago (legend has it she's been around since before Azir's time) making her the Voidborn who, so far, has dwelt the longest on Runeterra (she has spent all this time building a portal to the Void, using the life essence of her victims as fuel to open it).
Rek'Sai is not as sapient as the other Voidborn champions (she is to them what a dangerous animal is to humans).
↑Militarev, A. A complete etymology-based hundred wordlist of Semitic updated: Item 75-100, p. 179