Don't let the square shape fool you, Christopher's a force to be reckoned with. A telekinetic cube of unknown origin, he can turn the room freezing cold and induce paralysing fear. Although Marcus is the team leader, it's Christopher that acts as the Sparrows' consulted oracle who hands out incredible advice and serves as the family mediator. He's treated by the Sparrows like any other sibling.
As a general rule, I don't trust sentient cubes. But maybe Christopher will help bridge any gaps between the two Academy groups and lead to some teamwork to get the Hargreeves to their proper place in 2019. If they even have one after all this mess!