Okay I have a lot of headcanons/notes about sentient swords and I'm going to try and post them here so that I actually have a reference for the eventual attempt at drawing them that will definitely happen someday. Starting with 3zun's swords because those are the most extensive thoughts:
Always stands on her toes, leaning slightly forward with her hands back and tilted out like a bird about to take flight (thinking about. Rue from the Hunger Games intro description).
She's not actually blurry or soft, her edges are sharp, but she shakes? And vibrates at a frequency that seems to be slightly faster or somehow out of sync with her surroundings. This makes her hard to look at straight on, like your eyes can't pin her down, and so her edges all appear slightly blurred, or (falsely) soft.
You can look at her but as soon as you look away it's hard to pin down an exact shape in your mind. Body or features. It's just impressions - golden, fairly small, smiling?
Her skin is what initially appears to be liquid, shifting like the surface of a gas giant. Except, upon closer examination, it's actually almost every color mixed together, or ever-changing. It sometimes looks more solid and something like mother-of-pearl, but occasionally grows more liquid, the surface shifting faster and faster, during which it is tinted gold by a soft warm glow from underneath.
Eyes are large and without pupils, similar shifting quality to her skin, but various shades of deep red, like rubies, or blood. It's impossible to tell where she's looking, especially because she always seems to be tilting her head and standing at a slight angle, like an intrigued cat trying to pretend she isn't looking.
Short-medium (chin length? but extremely choppy and uneven, like it was torn or hacked off with a knife) black hair that floats slightly around her face. Almost looks like she's been electrocuted.
She has a face, and her own voice, which is soft and high pitched, birdlike, but not exactly soothing, as it shares the shaky quality of the rest of her. Except her true voice and form are rarely observed, because even when she is mostly herself, she ever so slightly takes on the appearance and mannerisms of those around her. The longer a conversation goes on, the more it focuses on one person, or the more intense it gets, the more she shifts to subtly be more like them. Or not. Maybe someone familiar, or something they'd admire?
She has wispy gold-ish tendrils that extend from her all around, though they tend to be more numerous and active when she's engrossed in something or otherwise distracted, but then immediately retract when she realizes she'd being observed.
The most solid wisps are two short pieces starting at her crown and twisting out above her head, pointed up and curled like horns or more like ears (they turn towards noises and occasionally go flat against her hair); a long one down her back that gets thicker at the end; a series of short spikes along her shoulders and getting longer down her arms until they're almost longer than the sleeves of her robes; and a series of feather-like wisps sitting over her fingers. Alltogether they make her look somewhat like a bird, or maybe a cat -- a very flustered great horned owl? When she's upset or startled the wisps can stick straight up or fluff out.
The more tendril like ones (which are less opaque and continuous) tend to slowly creep out around her, like feelers, trailing along surfaces and prodding at objects -- like they're scoping out the room, or looking for something. Very, very rarely, and only when she's fully distracted, they might twist along around someones ankle, or their wrist, or a piece of their hair. The second they move or try to look the wisp (which has no substance and can neither be felt nor actually hold anything or anyone) disappears.
She can spin thin silvery threads between her fingers, which she occasionally uses to tie small objects together, or weaves into webs. They're not very strong however, and slowly fade away after a few days, or sometimes when she leaves the room. She mostly seems to use the strings like a fidget toy, weaving shapes between her fingers like cats cradle. Very very rarely, she weaves little people or creatures out of the string, and leaves them to be found -- a gift.
All of her teeth are very sharp, with two longer ones on top like fangs. She doesn't have claws, but her nails are very sharp, pointed, and can harden into metal, growing longer and thinner until they almost look like needles coming out of her hands.
They don't appear most of the time, but she has glowing red eyes that appear on the back of her head, around her neck like a collar, and on the outer edges of both hands. It's not clear what they're for or whether she can see out of them, because her hearing is so sensitive she's impossible to sneak up on anyway.
She is solid. She is. She is. She's just hard to pin down, and the blurred edges and glitching and wispy tendrils don't make it any easier to get an idea of exactly where she is, or what she's doing. But sometimes, when she gets scared, it looks like she's teleporting, or popping very very fast in and out of existence, only within a short range of herself. Sometimes it looks like she splits into mirror images of herself all at once, letting out a squeak, before re-solidifying in the center into one person.
She moves very gracefully and lightly, tending to perch on furniture, but never sprawls, or looks fully settled down. Like an extremely polite but weary cat, that hasn't decided whether it's allowed to be here yet. Sometimes, she seems to be so wholly liquid that this grace makes her resemble something more like a snake, or a sentient river. This is especially true when she (very rarely) curls around people--perching on their shoulders before slowly relaxing into some sort of feather boa, or, running at them full speed to clamber up and twist around their legs.
She sleeps curled tightly into a ball, nose tucked beneath her arms and golden wisps curled around and around her until she looks like a ball of string with ears poking out. That is, if she was to be seen sleeping, which she isn't, favoring the highest nooks and crannies she can find, tucked safely away.
