User blog comment:Xilinoc/Ch. 555 - THE HERO/@comment-16214733-20131031162101/@comment-11051098-20131105224512

Well, when a human dies, its soul cross over to the SS. Souls can also be born within the SS. If the soul possess spirit energy/reiryoku, it can attain the Shin'o Academy and train to become a Shinigami. If a soul doesn't cross over to the SS, for various reason, its chain of fate will slowly corrode. When the entire chain and the base in its chest are gone, a hole opens in its chest, and the soul lose its heart and turns into a hollow.

Whether the soul of a Quincy goes to the SS when it dies, I don't know. The Fullbringers were shown to be in the SS after their deaths, apparently with their powers intact, so it's possible Quincies can do so too. If Masaki did, Yhwach could have found her. However, I don't think there's a possibility that he found her and made her a Sternritter.

He stole the powers of Gemischt Quincies when he performed his Auswählen, but he also stole the powers from Echt Quincies which he saw as impure. Masaki was an Echt Quincy, and lost her powers, probably because she was "impure" because she had a hollow inside her after White's attack. She also had Isshin's reiryoku and powers inside her, keeping her soul stabilized.

She lost her Quincy powers, and as far as I know, she died as an ordinary human, so I think she became an ordinary plus before crossing over to the SS.