Look between Saturn and the Monk, theres another line of shading. The giant is Imu, crawling down from the throne. Otherwise, the alignment just doesnt work. You can see Imu transforms right before that panel anyway
That thing?
Highly doubt it’s one of the Gorosei. All the others are clear to see with individual silhouette‘s and eyes. Would be weird for one of them to be this tiny little thing that can’t be clearly seen.
I doubt Imu has even risen from the throne. It’s more powerful an image for them to have remained sitting while all the chaos is going on around them.