Matteo while he was still a Sith apprentice.

Matteo is a Rutian Twi'lek who becomes an apprentice for Darth A-Idjh is the first book. He has blue skin with dark brown eyes and likes to use the Force to get what he wants. His last name is never revealed but he claims to be descended from Mission Vao. Unbeknownst to Darth A-Idjh, he is secretly a spy for Darth Sidious. Later he betrays Darth Sidious, by injuring him with a concussion grenade and robbing him.

Rumors spread among the Jedi that Matteo would be the one to slay Darth A-Idjh, as his Force sensitivity is incredibly high, fourth to Yoda, Anakin, and Carl. This rumor was completely true as he killed Darth A-Idjah and became Darth Kal.