Successfully complete the tasks indicated to unlock the corresponding character:
Elektra: successfully Complete Infinity Rift Range - Ultimate: Bad Business Lv. 30. This split is available for the first time in chapter 3.
Loki: successfully Complete Infinity Rift Sigma - Boss Wave: Defeat the Ebony Maw, and four Lv. 45 heads. This split is available for the first time in Chapter 9. Note: Loki is available only after you complete the game on difficulty 'Powerful'. To unlock it, you have to fight all five members of the Order Black in a process, one at a time, which is quite challenging.
Magneto: successfully Complete Infinity Rift Psi - Boss Wave: defeat Klaw and the two Lv. 38 heads. This split is available for the first time in chapter 5.
Thanos: successfully Complete Infinity Rift Lambda - the Ultimate: The Black Order Lv. 65. This split is available after completing chapter 10. Note: to unlock it, go to chapter 10 on Hard difficulty and complete many of the tests in the crack final. You have to defeat all five of the members of the Order, Black, all at the same time, to unlock it.