Regarding Fears and Traumas

 Mental turmoil is a huge theme in Silent Hill, and a number of the monsters that appear in both Silent Hill and Silent Hill 3 are related to Alessa's--and as an extension, Heather's--fears and tastes. Thus, the same can be said for Mizael, in that his fears and hates often align with those of the 'Astral' in his world, and thus, with the canon series itself.

Noted fears include:

• Fire; more than likely stemming from the incident experienced by Astral, an explosion of red and pain, he has a strong aversion to flame, and while the fear isn't paralyzing, it's clear that the sight of it brings discomfort. 
• Darkness; similar to Fire, this originates from the massive power that Astral absorbed in his sacrifice. This explains why Mizael carries a flashlight around, at any rate.
• Mirrors; known as 'spectrophobia', Mizael will admit having a dislike for them, but 'dislike' doesn't cover it when you cover the ones in your home--or at least the ones you can't get rid of. The fear could stem from the fact that mirrors are often seen as 'windows to the soul'--and thus, from a subconscious knowledge of who his past incarnation was, and more importantly what he himself has the potential to become.
• Dogs; a 'minor' fear, to the point where some mistake it for a mere dislike. He's shot quite a few mutated ones after all, and other than spending most of the time glaring at them, never seems to react any further to smaller breeds...

Of note among that which he detests, there is:

• Most insects; it's implied that this used to be a childhood fear, though he claims to have outgrown it. The noted exception to this are moths, butterflies, dragonflies, and those similar in appearance to them. He hates locusts the most, though overall, his dislike for insects is rather minor, and more an annoyance if anything. One could argue this relates to the 'Fearsome Four of the Barian World', as three of them were insects.
• Hospitals; potentially linked to Astral's own hellish experience, he doesn't enjoy seeking medical attention if he can help it, having a distinct lack of trust toward medical staff. He'll go if pressed, but will generally have a sour face the entire time, keeping a suspicious eye on just about everything. Hospitals and related could also be considered a minor fear, given that particular response.
• Worms, Snakes, Leeches, etc; overall if it's long and 'slimy' in any way he doesn't tend to want to be near it. He seems rather insistent on the fact that snakes feel different from other reptiles as well, so it's best not to question why he still likes even the more serpentine dragons. As it is, this likely was drawn from Don Thousand and Black Mist(Zexal)'s use of tentacles, as well as the only non-insectoid member of the 'Fearsome Four' (who was a jellyfish).

On the Plot of Zexal and the AU Proper

 One could wonder how this AU can be anything but a cut and dry character swap, but nonetheless, there you go. As such, events of note are listed below, in chronological order. Differences concerning characters during the events of the 'games' are not listed however--for those, they will be posted as the characters are 'revealed'. (So unfortunately, you still don't know if Yuuma's dead or not. ...Given the AU, you can assume a certain outcome.)

• While Kaito and Faker are indeed involved with the Zexal Plot, Kaito is 'age swapped'; he replaces Haruto's role in the series, while Haruto takes his as the 'Number's hunter.
• Events of Zexal largely happen as canonically seen, with exceptions being that Haruto, as Kaito's replacement, never encounters 'Mizael' (for good reasons). Because of this, he travels to the Barian World to seek out the Tachyon Dragon, an act that causes him to vanish even from memory.
• Of further note is that Gauche and Droite were never brought up as numbers hunters, with Droite eventually becoming a promising young nurse, and Gauche himself going into psychiatrics before everything went to hell. (Gauche, however, would be the only one to escape Heartland, due to his connection to Yuuma and the fact that he fled the city almost immediately after his death. This would spare him from becoming trapped in the fog world.)
• As it is--Don Thousand's plan to fuse the Barian and Human worlds begins, with Heartland at the epicenter. After reawakening Astral, Yuuma and his partner merge through Zexal in an attempt to stop this. However, Don Thousand is too powerful, and thus, they use another method.
• Taking upon themselves the massive darkness flooding the area, they cut the merge, leaving only Heartland City effected. The process splits Yuuma from Astral, and causes the latter massive agony as his body is made solid, the power threatening to over take him. To prevent their plan from backfiring, he casts off a part of his soul in the form of an infant, instructing Yuuma to take it out of the city.
• Though Yuuma does so, leaving the baby with someone who is fleeing with their son (this turns out to be Faker and Kaito), he returns to Heartland with hopes of saving Astral any way. Because of the flames devouring the city core, Faker eventually presumes Yuuma to be dead, adopting the infant as his own--he names the baby 'Haruto', after his presumably deceased elder son.
• Yuuma meanwhile did not necessarily die, returning to Astral's location to instead find Eliphas. As Eliphas moved to refuse him passage, viewing these events as proof of the boy's failure however, Astral's control over the darkness slipped--the world began to shift, and Eliphas, caught in the energy, was made human.
• Through the moments that followed, the Barians would be considered the Pyrrhic victors of the war, with Heartland City barely retaining its reputation as a vacation spot. However, as the power at Astral's 'control' affected the city, various alliances would shift--because of this, Heartland (the person) would enter Eliphas' employ, acting as the Doctor who oversaw Astral's ruined form, and ordering Droite, the nurse, to care for him directly.
• This would in fact be a double agent-like situation; while Eliphas had the intentions of renewing Astral in order to purify him of the darkness which tainted him, Heartland's own actions were to ensure that the darkness locked inside his body be intensified, effectively reviving Don Thousand through the emissary itself, and thus ending the Astral world's forces for good.
• The darkness however would not merely rely on 'Doctor' Heartland however--rather, some of it would take form again, as a second 'Black Mist'. This child would be discovered by Vector in the Fog World, helped to escape, and manipulated into leaving Heartland to meet with Haruto.
• The encounter would make the boy suffer from an unknown illness, one which Faker struggled to 'cure'. Only Kaito, as it seemed, would realize that the source was Haruto's 'new friend' however, and he would thus plead with his father to allow them to leave for a small 'vacation'...
• Kaito would in fact flee the country with his brother however, and for the next three years raise him on his own under the protection of the States, and a number of laws. Faker, during this time, would search in vain for his children, regretting what madness he had inflicted on them, while Black Mist would retreat to Heartland, eventually turning on Vector's plans of usurping 'God' and joining the very Order that would unwittingly revive Don Thousand, due to an indescribable desire hardwired into his mind.
• Faker would die when Haruto was ten, and Kaito 18, the latter only then gaining the right to officially care for his brother on his own. Stricken by guilt (as while Faker had 'lost it', he still had many pleasant memories of his father), Kaito would have a few days of upset, before Haruto offered a vacation to 'Heartland'.
• For you see, the evacuation of Heartland had been passed off as an explosion. After the initial merge and battle, the city settled back into the illusion of the normal city for the most part--at least from the outside. Quieter, and a little foggier, but no less the 'vacation' it was supposed to be, right?
• Kaito would be hesitant. However, Haruto insisted, and so they went. They took a plane to a city outside of Heartland (as it would be much cheaper), and rented a car to drive to the city itself.
• And as a white-blue figure appeared on the road there, they would crash...