Michaela is an alias to refer a specific kind of vampire named, and possibly, created by a second progenitor known as Saitō. This feature is explored in Seraph of the End: The Story of Vampire Michaela.
The Michaela trait only appears in one out of tens of millions of boys prior to the Apocalypse. After the Apocalypse, it is nearly impossible to find one.
Known People with the Michaela Trait
What It Means
Kagami-sensei may tell us in future volumes.
The story goes back to the origin of vampires when the angel Michaela fell to the earth.
Starting about 150 years before Crowley meets Ferid, Ferid watches and waits for him to be born. During the events of The Story of Vampire Michaela (Book 2), Ferid uses Saito's blood to turn Crowley into a vampire during the 13th century. He does not mention the term Michaela to Crowley, but he tells Crowley he was watching for him long before he was even born. He says he uses Saito's blood instead of his own in order to make Crowley much stronger and more akin to a brother than a servant.
During the 21st century, Ferid manipulates Krul Tepes and Mikaela Hyakuya and manages to have Krul turn Mika into a vampire. Mika is also one of the Seraph of the End, who was previously experimented on by the Hyakuya Sect.