At the request of another Twitter user, I present, Axiom. Son to Zeta Prime and Sentinel before the fall, he was predicated to be the next Matrix bearer due to the marks on his chassis and the Primely height he inherited from Zeta. With this in mind, he was raised accordingly and considered a Prime in all but name.
By the time Sentinel performed his coup, Axiom was mere days from being declared a full blooded Prime. He never recovered from the loss once he was informed of the fall of his entire family. But upon seeing Sentinel's corruption and learning the truth, he isolated and eventually vanished.
That choice ultimately saved his life.
Sentinel never was one to have his authority challenged. Even by his own son.