@peony-pearl. Here is like 15 headcanons that I came up for KH's Seph, but I will say that these may be a little inaccurate since I've only played through KH 1 and 2 only once. ☺️
If I do add my KH AU to the official FWC multiverse, I'll most likely have some of these.
Unlike its standard depiction, his Masamune does not have a tangible weight in the physical world. Because it is forged from Cloud’s lingering guilt and dark memories, the blade feels lighter or heavier depending on Cloud’s current mental state. If Cloud is close to finding the Light, the sword becomes nearly weightless and impossibly fast.
His single black wing is not a physical biological trait, but a literal shadow cast by his heart that has taken material form. When he enters areas bathed in absolute, pure Light (like the highest tiers of Olympus or Castle Oblivion), the feathers faintly smoke and dissolve into dark vapor.
He has no natural memories of Radiant Garden, yet when he walks through the ruined corridors of Ansem’s Dark Depths, his hand instinctively traces the walls. He experiences phantom echoes of things Cloud witnessed there, mixing their consciousness in confusing, disjointed fragments.
The Heartless completely ignore Sephiroth, and he never commands them like Maleficent does. Because he is already a highly concentrated, sentient manifestation of a specific person's darkness rather than a fallen heart, the Heartless perceive him as an environmental force, like a localized storm, rather than prey or a master.
Sephiroth doesn't sleep, but when Cloud enters a deep sleep or a healing stasis (like Sora's pods), Sephiroth enters a state of absolute sensory deprivation. He stands perfectly still wherever he is in the worlds, hearing nothing but a low, rhythmic hum that matches Cloud's resting heartbeat.
His iconic Heartless Angel ability isn't just a magical curse. It is a brief, forced telepathic projection of Cloud's worst nightmares into the target's mind. The sudden drop to 1 HP represents the target's heart temporarily freezing from psychological horror.
The meteors he summons during his ultimate attacks are actual fragments of fallen, forgotten worlds that were swallowed by darkness. He pulls them down from the Lanes Between through localized rifts, weaponizing the debris of the cosmos.
He holds a deep, quiet contempt for Organization XIII. He views the Nobodies as pathetic, hollow shells trying to mathematically calculate emotions they no longer possess, whereas he views himself as the purest form of raw, unfiltered human emotion given flesh.
Unlike others who use the Corridors of Darkness and must wear protective armor or black coats to protect their hearts from being consumed, Sephiroth can walk through them completely unprotected. The raw darkness of the lanes actually rejuvenates his strength, acting like a natural element to him.
When he "vanishes" after being defeated or after his clashes with Cloud, he doesn't travel to another world. He dissolves back into the dark recess of Cloud’s subconscious mind, waiting for the next moment Cloud experiences a drop of self-doubt to manifest again.
He is entirely indifferent to human comforts like food, drink, or warmth, but he is highly sensitive to sound. He despises the noisy, chaotic atmosphere of the Olympus Coliseum crowds, preferring the absolute, echoing silence of the dead spaces beneath Hollow Bastion.
If someone other than Sephiroth or Cloud tries to lift the Masamune while it is resting on the ground, their hands pass directly through the hilt as if it were a mirage. It only exists in a solid physical state for the two people tied to its origin.
He occasionally speaks in cryptic lines or references events that happened in worlds Cloud visited but Sephiroth himself was never present for (such as the details of the underworld or the fall of the prompt world), proving their minds are constantly leaking into one another.
Sephiroth finds the Keyblade deeply fascinating but fundamentally flawed. He views it as a clumsy tool that forces a choice between Light and Dark, whereas he believes true power lies in completely mastering the inevitability of one's own shadow.
He doesn't actually want to kill Cloud. If Cloud were to truly die, Sephiroth would instantly cease to exist, as the anchor creating him would be gone. His true goal is to keep Cloud in a state of permanent, perpetual conflict, ensuring they both live forever in the twilight of an endless hunt.
