Because it could be important!
Characters change throughout the course of games, and of course, one person's headmuse will differ from another's headmuse of the same character. This post outlines my own personal headcanon for my Deidara.
The first section is overall headcanon for the muse himself. This usually stays consistent regardless of where I'm playing or writing him, though the entry will mention how the headcanon directly applies to Facility if applicable.
The second section is for headcanon that's only come into play at
a_facility; things that change the Deidara you'll see in the game from the Deidara you see in the series. Both lists will be updated as more headcanon presents itself to me, or as more changes occur in-game.
General Headcanon
1) Deidara wasn't born with his hand-mouths and chest-mouth. They're a gruesome side-effect of the forbidden technique he stole before leaving his home village (the one that allows him to knead his chakra into things like clay). Horrifying physical transformations like this are probably why the technique was forbidden in the first place. And why the Deidara after his current timeline for this game still had a hand-mouth in the replacement left arm Kakuzu stitched on. Considering some of the things he could have ended up with, Deidara's not really complaining.
2) Kurotsuchi is not his little sister; she's a close friend he had before defecting, and one of the only things about the place he misses. (Her calling him "Deidara-nii" in the manga could just as much be a sign of respect as literally calling him big brother; I choose to go with the former.)
3) Being as close of friends as he was with Kurotsuchi, the two of them shared techniques and training. Deidara knows more about kunoichi wiles than most males have the right to consider. This includes how to see through and ignore them... as well as how to employ a few. Like seduction. And the blatant use of sex to manipulate.
4) Despite this, Deidara is still an artist at heart. He has more depth of spirit than most people give him credit for, and deep down inside, he's a hopeless romantic. He's usually very good at hiding this, though it does manifest in pangs of jealousy and a very low tolerance for betrayal.
5) When gathering the dirts and compounds he uses to make his clay, Deidara would often come across a variety of creepy-crawlies working their way through the raw materials. He developed a sort of respect for the things living in the components of his art, which is why he sculpts so many insects, arachnids and arthropods in his works.
6) Besides this, Deidara harbors a deep, lifelong, childish desire to have his own wings and be able to fly without aid. He loves birds even more than he loves bugs, and has spent hours upon hours studying them. His bird sculptures are always as graceful as he can possibly make them, and he varies the species of his flying mounts as a sort of homage to the creatures he's seen. This is also part of why the exploding doll seen for his C3 level attack has wings for arms, despite its inability to fly.
7) Deidara will wax nostalgic on art until he's blue in the face. Any artist that uses any medium at all, anything that could even be broadly considered art, gets infinitely more respect from him than any non-artist. He will even stay his aggression against a person to philosophize and otherwise just talk art with him or her instead.
8) Deidara didn't defect from his home village only because he had no choice after stealing one of its forbidden techniques. He hated the place and most of the people in it, particularly the crotchety old kage. Despite the praise they gave him for his sculptures, he considered most of the inhabitants to be uneducated idiots when it came to the depths that art should have (whether or not this was actually true).
9) While he doesn't mind the occasional sweet, Deidara much prefers savory. Salty things are his favorites, and he's known to mix salty and sweet or to outright salt sweet things to make them "better." He salts his tea.
10) Never established in canon one way or the other, so: my Deidara can make non-explosive clay. He uses non-typed chakra (the same sort used to make shadow clones, transform, use escape jutsu, walk on walls/water, etc.), so the sculptures aren't volatile, but he can still mold them into whatever shape and control them remotely. For practice. Or when bored out of his head.
11) Deidara sucks at cooking. He sucks really hard at cooking. He's too impatient to be any good at it. He tends to forget little important steps like putting oil in the pan. And cooks everything too hot and too fast. And breaks utensils because he doesn't bother to switch from a scraping spatula to a mixing spoon.
12) Deidara is claustrophobic. He really loves open spaces, open skies, and flying; he's also a long-range fighter. Being somewhere as closed as the Facility makes him skin-crawlingly nervous. It's not nearly as bad as it could be, though; if he were locked in a closet, he would probably panic. This fear also partially translates to being held down or otherwise restrained. He's okay when using his mole-tunneling technique, only because he's completely able to move around at will through earth and clay when using it.
13) Deidara knows a little more about the Uchiha clan than your standard non-Konoha shinobi. He doesn't know their guarded secrets, but anything one can pick up in rumor, conversation, or non-classified files (and maybe a few stolen classified files), he's privy to. Best to know one's enemy.
14) While he has few qualms about killing people, is completely un-bothered by burns and cauterized wounds, and harbors no fear of death, Deidara is kind of grossed out by bloody gore. Wide-open bleeding wounds earn a bit of a lip curl, while organ exposure gets a loud and mildly flaily expression of heavy disgust. It's not a fear, and it won't ruin him for a fight, but he certainly doesn't want to hold a heart in his bare hands.
15) Deidara has no fears of death, so long as it is a quick death. He is likewise unafraid of fire, explosions of any kind, structural collapse/being crushed to death, and most kinds of swift bodily harm. However, he is terrified of slow deaths, particularly forms of suffocation (smothering, choking/being choked, drowning, etc.).
16) Being something of a masochist, Deidara has little fear of pain. He does not enjoy all pain, but neither is he particularly bothered by it. He has a very high pain tolerance, and most tortures that will cause pain but leave his body largely intact will be completely ineffective in breaking him.
17) Deidara is commitment-phobic. He lives every moment as though it might be his last, and enjoys every day like a dying man. He makes no commitments both because he sees no use for them and because he has no interest in making promises that death will just break. It's a very easy way to live. The idea that he could live forever in Fac terrifies the hell out of him.
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**) [Note: This one is touchy. I only use this when I feel it fits what and where I'm writing, as this is very much headcanon and has absolutely no evidence for it in the series.] Not long before defecting from Iwagakure, Deidara was sexually assaulted by his squad leader. He ended up killing the other man in order to protect himself: he jabbed a sculpting knife through the other man's jugular. Deidara's initial yells for help before managing to stop the other man prompted authorities to break in in an attempt to intervene, at which point they found a terrified and half-sobbing Deidara partly pinned under his team leader's body and covered in the other man's blood. The evidence in his favor was obvious and he never even went to trial, but his teammates became wary and distrusting of him nonetheless. This incident is why he's generally distrusting and resentful of (if not outright hateful toward) authority figures, and part of why his tolerance for betrayal is slim to nil.
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Facility Headcanon
1) Deidara hates being manipulated. The fact that he's been dragged into Facility against his will and with no chance of retaliation in the first place makes him even more surly and hateful than usual.
2) As a result the
public breakdown he had during psych week, Deidara's pride took a hard hit. While he tells himself that he spent several weeks in hiding to avoid being killed by other Facility subjects, truth is, more than half the reason is shame from having been so weak in public.
3) Deidara feels incredibly vulnerable in Facility. Its closed-off nature freaks him out claustrophobia-wise. He's also stuck in an enclosed space with people who want him dead, something that has never before been a problem. The inability to escape, either by tunneling or by flying, makes him a little bit skittish.
4) Regarding the "love" kanji tattoo on Sasori's head during Deidara's
broadcast dream: Gaara physically confronted Deidara during psych week, and was partial cause of and witness to Deidara's breakdown. However, he didn't kill Deidara, nor did he later bring up the event. He was also willing to talk to Deidara on
a post of his own, with calm and civility. Between the madness of the previous weeks and his years in Akatsuki, that level of civility is associated in Deidara's subconscious mind with comfort and security. So, some small part of Gaara got featured in the comforting part of Dei's dream.
5) Deidara has recently become much more polite in general than he normally bothers with. His speech patterns are moderately more carefully-formed than usual, and he is much more prone to drop a quick "thanks" or "sorry" than his canon counterpart. This is part reversion to traditional Japanese politeness and part careful self-editing. All of it, however, is in response to being stuck somewhere where he could be killed at a moment's notice, with no escape route -- but where he'll just come back again to face those already angry with him. And very nearly all of it is a front. Better to feign politeness than to get himself murdered in his sleep repeatedly.
6) Deidara, having been brought to the Facility from a point at the very beginning of Shippuuden, is not aware of the bijuu plot or any successive plans of Akatsuki. He's heard murmurs and rumors from Pein and Konan about the subject, but nothing like a solid plan. He was not, upon arrival, aware that he and Sasori were to capture the ichibi jinchuuriki, nor in fact that any sort of capture/extraction was part of the organization's agenda. He's only slowly learning more about that particular bit of insanity.