A Demon Among Devils by Crimson Lord is a crossover of the High School D×D universe with the Persona3.Minato Arisato woke to find that Elizabeth had freed him from the Great Seal and dropped him off in this new world, where devils, fallen, and angels dwell. Accepting his circumstances, he settled into his ordinary life within it until Issei came to him and asked what he would do if a friend was in danger. From there he found himself standing before the fallen, Raynare, who passed her judgement on him.Refusing to be judged, he summoned the power of the Persona he carried within the Sea of Souls and thus began the story.
Takes the cake in chapter 13. First, the Knights Templar arrives chanting a revised version of their creed to help them. Then came the three Fallen Angels that were with Raynare. Then Minato himself arrive. And at the end? The 7 strongest Archangels - Metatron, Sandalphon, Raphael, Michael, Gabriel, Uriel and Melchizedek - appears on the battlefield themselves.
Chronic Hero Syndrome: Minato has a tendency to help not only those he avoids, but also his former enemies. He allows the Fallen Angels to live due to their chance at redemption for their lives, he offers to help Rias avoid the Arranged Marriage despite how he initially had no reason to help, and he heals Riser's peerage after they were all gunning for him during the Rating Game.
During Riser's Rating Game against Rias, it's noted that many of his pieces are actually inferior in quality when compared to Rias's and he only manages to gain the advantage thanks to leveraging his superior numbers to separate and take down each of her pieces individually.
Outright inverted with the Knights Templar who's holy blessings are explicitly more effective the more of them there are.
Curb-Stomp Battle: So far, everyone who has directly battled a Persona is on the receiving end of one. Understandable, considering many of them are just ordinary entities fighting their equivalent of living legends.
Minato gives one to Issei during their match after equipping Nidhogg to himself.
Kokabiel delivers one to Kiba, Irina, Tsubaki and Xenovia so badly that he almost kills them even though he was clearly just playing around. Asia is the only reason they manage to survive to regroup and they opt to temporarily retreat rather than fight him again.
The Archangels and Thanatos quickly turned the tide against the two-hundred or so Fallen Angels.
Darker and Edgier: From the beginning it goes straight into this territory. There's still comedy, but the enemies are much more serious threats and the battles far bloodier.
Dare to Be Badass: Nidhogg encourages Issei this way, challenging him to work until the day he is worthy of inheriting Ddraig's power.
Despair Event Horizon: According to Raynare, all Angels that become Fallen experience something similar to this during the process. Usually, the Fallen will eventually come to terms with it to some extent and some, typically those that choose to fall willingly, even come to revel in their Fallen status.
Although they personally aren't enemies, when Xenovia and Irina are sent to retrieve the Excaliburs they enlist the help of several devils and Minato, due to knowing that they couldn't do it on their own.
Though reluctant, Rias and Sona's group ally themselves with the Knights Templar and later a few Fallen Angels in repelling Kokabiel's forces.
Epic Fail: Issei gets his thunder-cannon to boost up to the max for the first time and aims it towards Kokabiel, with everyone thinking it could mean victory for them... He misses, without the Fallen Angel bothering to move.
Fear Is the Appropriate Response: Thanatos prompts this from everyone. All the Fallen Angel were willing to fight the Archangels as brothers-in-arms, knowing that death would be the end. But Thanatos they ran from futilely and throwing each other towards him to try and escape.
Dohnaseek, Kalawarna and Mittelt are each given a mark of penance by Metatron which would allow them to redeem their status as Angels, they eventually help out and betray Kokabiel when he assaults Kuoh Academy.
The Knights Templar aid the devils in their defence of Kuoh Academy after Uriel defeated them in the cathedral. Since they're religious zealots, it makes sense.
Raynare herself eventually does this after Minato brings her back to life.
Knight Templar: Literal Knights Templar, trained exorcists who act in a unit to overpower even high-level devils.
Metaphorically True: So far Minato has deliberately left out information about him being from another world when talking about his past. When Kiba surmises that SEES is overseas when it’s mentioned that they are very far away, Minato says it’s “something like that.”
Not Always Evil: Minato was originally thrown off when he finds that Rias, Sona and their Peerages are all completely normal, nice and friendly people due to his experiences with his Demon Persona. As it turns out the original 72 Pillars were Always Chaotic Evil and obscenely powerful, ruling over the underworld with an iron fist. However when all the originals were killed off during the Great War their descendants were given free reign to rule themselves and many decided that they no longer wanted to be the kinds of devils that lived for hatred, evil and destruction and rebelled.
Offscreen Moment of Awesome: Happens regularly. Typically whenever Minato summons the Persona of an extremely powerful and famous mythological figure, once they’ve finished their entrance and are ready to open a can of ass kicking on their unfortunate enemies, the next scene cuts to the direct aftermath or even to events well after the battle. In some cases there are anachronic scenes that go into what happened directly after the Persona is summoned but they typically end before the actual battle begins.
Oh Crap: Anytime anyone see's a Persona summoned against them for the first time, partly due to the fact that Minato tends to use ones that are significant to his current opponent in some way.
Priceless Paperweight: Minato uses the Evil Pieces given to him… as ordinary chess pieces on a board. Subverted with the Excalibur pieces that Michael gives him who thinks he plans on doing this, instead Minato gave Xenovia and Irina one each and Kiba the remaining two.
No matter where Minato goes or where he tries to hide, Issei will always know exactly where to find him at any time during the day.
Sometimes Minato returns to his home ready to relax after a long day only to find someone already inside who wants to talk to him about something important. Becomes less funny at the end of chapter 14 when said people are the potentially hostile Hero Faction.
Spared by the Adaptation: Raynare as well as the fallen who were with her in the first chapter don't snuff it in the beginning. This is because Metatron wants to restore the heavens and all of them were capable of being redeemed... well, not Raynare initially, but Death Equals Redemption.
Stating the Simple Solution: During the planning process Rias and her Peerage go through in determining how they should use Minato’s strength in the rating game against Riser, Kiba points out Minato could simply summon Nidhogg and have him blast Riser and his peerage to kingdom come straight off the bat. This is shot down due to the fact that a) normally Minato can only comfortably summon a single Persona and was currently using Belial to fool Rias’s Evil Piece and b) Minato is still doing his best to remain under the radar of the Three Factions which wouldn’t happen if he summons an infamous Evil Dragon in broad daylight.
The Stoic: Minato. The only time he actually gets truly angered and threatens to break this state is when Raynare is killed. It takes him some effort to buckle it down.
Tempting Fate: Kokabiel mocks them when they claim Minato was going to stop them and asks where is he now then. He raises his hand, revealing he's already there.
The Power of Friendship: The Social Links that Minato made in his own world with SEES is what allowed him to gain so much power in his Persona. It’s also implied that the Social Links that Minato can form are also having an effect on those that he is becoming close to though it has the inverse effect of powering up Minato’s friends rather then himself.
Took a Level in Badass: Issei gained a thunder-cannon, Kiba gained the ability to create Nihil Weapons, and Raynare gained Metatron's legendary spear after being reborn as an Archangel.
Trapped in Another World: Minato in a sense. Though he does still live in Japan, there are enough differences between his world and the Highschool Dx D one that he quickly realises that he’s not actually in his home world but in some sort of Alternate Reality.
True Companions: Minato and SEES were this. Most peerages seen so far also fall into this with Rias, Sona and Riser’s peerage’s all demonstrating loyalty towards each other.
Villainous Rescue: Twice in chapter 13. First when the Knight Templars arrive to help rout the Fallen Angels attacking Kuoh academy and later when Dohnaseek, Kalawarna Mittelt arrive and manage to distract Kokabiel long enough for Rias and Sona to escape him.
When Belial wipes out the rest of his Peerage, rather than try and escape again Riser attempts to fight back against Belial with a massive blast of fire. He accomplishes nothing but standing firm and trying to take on one of the original Archdemons definitely takes balls.
The remains of Kokabiel’s Fallen Angel army are all veterans who immediately began working together flawlessly against the 7 Archangels Minato summons even while knowing full well they stood no chance.