Aggregate Will (意思総体 Ishi Sōtai) is an individual's consciousness, one of the components in making them an individual. It is thought to be similar to the concept of a person's spirit. Descriptively, an individual is formed when Aggregate Will dwells within Power of Existence.
Background[]
With their Aggregate Wills swapped with each other's due to the effect of the Treasure Tool Reshuffle, Yūji Sakai struggled to manifest Yogasa in Shana's body, whereas Shana was easily able to cast a Fūzetsu in Yūji's body. When Shana cast the Fūzetsu in Yūji's body, she snatched Power of Existence from her own body, which had Yūji's Aggregate Will inside, while her Aggregate Will was still in Yūji's body, and used it as the energy source for the Fūzetsu. Therefore, the cast Fūzetsu's color was crimson.
When the Crimson Lord "Commander of Thousand Conquests" Orgon leads his Unrestricted Spell Legion, a countless number of military forces made of paper and a manifestation of his own existence, his hat and cloak, which his Aggregate Will dwells in, acts as a control tower. The Crimson Lord "Destructive Blade" Sabrac's existence was exceedingly thin and enormously spreading, and when his humanoid form, which had his Aggregate Will inside, got too far away from the rest of him, he would lose control of the larger body he had left behind.
In other words, memories, character, essence, experience, idiosyncrasies (Flame color), and other such things are a part of Aggregate Will. However, it is possible for Aggregate Will to be separated from Power of Existence and the physical body, as well as any special characteristics attached to them.
Enveloped deeply in the mysteries behind the Perfect Insignia Psalm of the Grand Order was something called the "Virtual Aggregate Will" (仮想意思総体 Kasō Ishi Sōtai). Over many years, many personality reflections (a human's strong emotions, their personality patterns) were harvested by Tyrant II's Reflective Transfer and were to be combined and completed by the Psalm of the Grand Order's final Insignia into a literal temporary Aggregate Will which the Snake of the Festival would be able to resonate with and move at will while still in the Eternal Pitfall. After unifying with Yūji Sakai, the Snake of the Festival's Aggregate Will was able to surface. After Bal Masqué had accomplished the Grand Order, Bel-Peol's Treasure Tool Tartaros separated the Snake of the Festival's Aggregate Will from Yūji Sakai and he departed to the New World Xanadu.