They explained everything about DV's apprentice months ago on gametrailers tv.
He is trained to kill all the other jedi's who were still alive between Star wars episode III and IV.
If you can remember Yoda sended a message to the Jedi's at the end of Episode III that they have to hide and never come back to the Jedi Temple. You never really know what happens with them only that almost every Jedi died when Luke mets Kenobi.
I think it is him though but DV had a bunch of other apprentices:
Ahsoka Tano, Jedi Padawan of Anakin Skywalker before he became Darth Vader.
Tao, a former Jedi Padawan, and one of the earliest students of Darth Vader.
A Secret Apprentice named Starkiller, a Dark Side Adept who was selected by Darth Vader to help him during the Great Jedi Purge.
Kharys, a S'kytri who was discovered by Vader during the Clone Wars. Vader promised to train her after the wars were over and kept his promise even after his fall to the dark side.
Lumiya, an Emperor's Hand that eventually became a student of Darth Vader as his potential Shadow Hand.
Flint, the son of a Clone Wars Jedi, Flint Torul, and eventual stormtrooper. He was eventually selected by Darth Vader to receive exclusive training as a foil to Lumiya and potential Shadow Hand.
Rillao, a Firrerreo Force-sensitive who was delivered to Vader by Antinnis Tremayne for training. However, she resisted the lure of the dark side and fled from the Dark Lord's service.
Hethrir, a Firrerreo Force-sensitive and mate of Rillao who was more susceptible to the call of the dark side, eventually becoming the Procurator of Justice.
