Saitou has taken to following her, not unused to the position considering partnering with Souji is more often than not letting him lead them around and Saitou merely doing some creative and subtle backseat driving when it's felt necessary. He doesn't do the same for Chizuru, but follows her all the same.

And although he looks back at the rumbling first rather than up or down, he doesn't linger when he notices the staircase starting to fall apart. He turns about the time Chizuru gasps and yells for him to make haste and there is no pause for a second thought, following her up the stairs and if he finds her step just a little too slow in terms of simply not being used to maneuvering like this on such short notice, Saitou will simply without question or a skip in his movements, scoop her up into his arms to take her up the stairs that much faster if necessary. Either way, there's not really an obvious option for them if they fall and if this is the only way through and they lose this opportunity... well, they just can't let that happen so there's no real use thinking on the consequences of it.

And Saitou has no plans of making Chizuru go on by herself by having only one of them make it up to the top.