The Ultimate Self is a concept that describes the full totality of an individual being across all of Paradox Space. Where as an individual self represents only one instance of a person in their specific timeline and manifestation, one's "Ultimate Self" encompasses an amalgamated whole of all such individual instances of the self, their memories and experiences. Davepetasprite^2 describes the ultimate self as “a superceding bodyless and timeless persona that crosses the boundaries of paradox space”, and which, due to its distributed nature across infinite different timelines, is a truly immortal version of the self. The Ultimate Self also reconciles an individual with experiences that would seem to be lost to them due to timeline manipulation, via traditional time travel or via retcon mechanics.
A realization of one's Ultimate Self can be achieved by attaining the deepest possible understanding of one's Aspect. This is said to be possible for Sburb players of all mythological roles, and may be the ultimate realization of God tier abilities, although particular aspects seemingly grant an easier route to understanding one's ultimate self. Davepetasprite^2 gives the examples of Heart and Time as two such aspects, as the former is involved with various manifestation of the soul and self, whereas the latter incorporates concepts of managing various manifestations of the self across timelines. As these two aspects are those of Davepeta's two components, this partially explains their particular knowledge of the concept of the Ultimate Self, although they also attribute this to the advanced insights into reality that come with being a Squaredsprite.
The Homestuck Epilogues Edit
The concept of the Ultimate Self is crucial to the plot of The Homestuck Epilogues. This post-Canon work deals with the consequences of characters coming to realize their Ultimate Selves. This is a process that is referred to by Rose as “ascension”. The way in which this manifests varies from individual to individual. In most cases, the realization of the Ultimate Self results in the deterioration of the body, as the physical body and mind cannot endure the "unbundling" of the experiences and processes of the Ultimate Self. In the case of Rose, her increasing realization of her Ultimate Self manifests a debilitating series of visions and accompanying migraines, which reduce her to a weakened and frail state. As this "condition" progresses, she becomes increasingly detached from her body, eventually only maintained by the intervention of Dirk Strider, and, later, through the shifting of her consciousness into a robotic body.
Dave Strider is able to unify with his Ultimate Self in a single moment, due to the intervention of the Hope powers of a holographic manifestation of Barack Obama. This kills his body instantly, requiring his consciousness to also be transferred into a robotic body.
Conversely, Dirk Strider's own realization of his Ultimate Self is characterized by a powerful integration of his disparate selves, and beyond this a dissemination of elements of his self across the scope of the Homestuck canon. This is likely due to his role as the Prince of Heart, an aspect principally concerned with fragmented selves and personas. "Ultimate" Dirk is able to leverage his distributed persona to "become" a narrative voice within the story, on a metatextual level, an ability that allows him to re-write reality and rival Alternate Calliope, a fully realized Muse, one of the two master classes, for influence over the narrative.
Ascension and Mythological Roles Edit
The differing approaches to realizing one's Ultimate Self seem to be intimately tied to one's mytholgical role, and this can be seen to some extent in the manner that characters describe the experience of "ascension". Rose, a Seer of Light, describes her ascension in terms of visions and an increasing awareness of the self and reality; “As I approach the realization of my Ultimate Self, I cannot stop the extant knowledge from seeping in. I am plagued by near constant visions from the less fortunate versions of myself, as well as a broadening view of the metatextual nature of our existence. Day by day I get closer to comprehending the full picture of the narrative.”
Meanwhile, Dirk's description of his Ultimate Self from the perspective of a Prince of Heart is more concerned with the breakdown of the barriers between his alternate personas; “Your Ultimate Self, that which is revealed when the mind’s partitions are stripped away, and all potentiality of who you are and what you could have been flow together.”
Jade Harley, as the Witch of Space, describes her own Ultimate Self in terms of multiple Jades in different "places" throughout the universe and her timelines; “like... one ultimate self distributed across multiple bodies. so in multiple places and states at once. every jade that exists is like a light being shined through a thousand cracks in the timeline”. It is worth noting however that Jade had seemingly not "ascended" or unified with her alternate self when giving this interpretation, although her perspective on the concept may still be useful for drawing a through-line.
Finally, Dave's ascension to Ultimate Self realization is described in terms of his visions of multiple timelines; “Infinite experience flows through his consciousness, an unimpeded torrent of raw potentiality. He sees everything. The roads not taken, the lives not lived, the thoughts, desires, fears all unacted upon.”
Speculatively then, given these varying descriptions, it seems that the process of coming to terms with and amalgamating the "Ultimate Self" is thoroughly governed by one's class and aspect as the framework through which the Ultimate Self is sought, and this informs the manner in which the understanding may manifest, as well as the specific physical and metaphysical effects of this understanding and realization.