Let Ragna idle during a fight with Terumi and you hear this...
"They won't find your body.
As of Chronophantasma, he gains a new one in the form of Celica. Harm her in his presence, and they'll be cleaning up what's left of you with a mop. He nearly flipped when Azrael attacked Celica in an attempt to force him to activate his Azure Grimiore. And he nearly strangled Kokonoe when she wanted to use Celica for Kushinada's Lynchpin with the full knowledge it would kill her.
Jin was neglected by Ragna when Saya became ill for reasons currently unknown, and felt resentment towards her for stealing his nii-san away from him. Any who share her face (like Noel and Nu) are automatically treated worse as a result.
On that subject, he will do anything it takes to get you out from between him and Ragna. In The Wheel of Fortune, Tsubaki learned this the hard way.
Jin also has feelings for Tsubaki Yayoi, though he normally chooses not to admit them — if you feel like hurting her, your death will be cold, slow, and very painful indeed.
Noel has always felt... inadequate around her peers. Don't bring it up.
Don't call her stupid, either.
Feel like hurting Tsubaki? She'll gun you down without a second thought. Terumi uses this as his means to turn her into Mu-12 in Continuum Shift.
Don't mistake her for a boy. Bang and Amane learn this the hard way.
While Rachel is usually quite condescending to those around her, she means well, especially towards Ragna, who she brought into the conflict. Best to leave that unsaid unless you want to stand in a lightning storm.
Having been reconstructed by Kokonoe, Tager's loyalty to her knows no bounds. Disrespecting her isn't something he takes lightly, so if you decide to badmouth her in earshot of him, pray she calls him off.
In Chronophantasma, he also gets pretty pissed and is unable to focus on his mission when he sees Bullet getting beaten up. That's because he and Bullet used to be part of the same mercenary group before his entire squad was wiped out, but he has no memories from before he was rebuilt by Kokonoe, so he doesn't even know why he's mad.
Litchi Faye-Ling and Roy "Arakune" Carmine were close colleagues during their tenure in Sector Seven, and when the latter tampered with the Boundary and wound up... disfigured, Litchi quit her job and went out to search for him. The more you insist she leave him to his fate, the more willing she will be to silence your complaints. This includes physical force.
One of the reasons Arakune became what he is is due to his inferiority complex to Kokonoe, something which Relius Clover fostered. His trip into the Boundary to tamper with the Azure drove him batshit insane — mentioning Kokonoe to him (or, as of Chronophantasma, her being around him) turns it a whole new shade of ugly.
Bang is loyal to Lord Tenjou to the end, and one of his lifelong goals was to bring his killer Jin Kisaragi to justice. He was willing to fight Jin on sight until Platinum talked him out of it. He will postpone the fight if Jin is injured, however.
If Bang believes you are a threat to his beloved Miss Litchi, he will beat your ass into the ground by any means necessary. This is usually gag reel material, but events in Chronophantasma have given this frenzy switch a more serious turn.
Do not hurt a child in front of Bang for any reason at all. He will do anything in his power to make you stop, even if he must break his vow to Lord Tenjou to make it happen.
If he suspects you know anything about Relius' whereabouts, you have three choices: tell him, get the fuck out, or die.
Ada "Nirvana" Clover
Nirvana doesn't really show much sentience, but just as Carl is fiercely protective of her, she's is equally protective of him. Harming Carl in her presence is basically wearing a "Kill Me" sign on your neck and will most likely be met with Wolverine Claws to your throat.
Hakumen cares dearly for Tsubaki Yayoi due to him being a past incarnation of Jin Kisaragi. Do not harm her unduly in front of him. You will die.
Hakumen also hates Terumi for the bullshit he pulled with the Six Heroes in the past, and intends to kill him over it. By the end of Chronophantamsa, he succeeds.
Between the Azure Grimoire's urgings to become whole and the lingering fragments of Saya's personality within her, Nu will do anything to be with Ragna. Do anything to interfere (being female is enough), and you will be skewered. Right, Tsubaki?
Tsubaki grew up lonely with House Yayoi, and only Jin got to interact with her. As a result, she loves him with all her heart. She can be quite vocal if you try to get between them, and as long as she's dirtied from Terumi's Mind Rape,she would kill her best friend over this.
On the subject, don't harm Jin in front of her, or she will turn on you. Try to force her into harming him, and she will resist you. Even if you're Izanami.
Hazama / Yuuki Terumi
Terumi's Pride is the core of his rage, as his ego bubble is the size of Ikaruga itself. The more you damage his overblown self-image, the more violent he will become with you. In order of increasing magnitude:
If he wants to torture someone for whatever reason, don't interfere. Best case, he will swear his head off. Worst case (like if you stick around), he will turn his knives on you.
Feel like using his own words against him or directly attack his self-image? He skips straight to violence. If he can kill you, he probably will unless you hightail it out of there.
Terumi hates being a tool for someone else's designs. If you put him under your control for any reason at all, that leash better be on tight if he's aware of it. He manipulated Trinity into having Mind Eater lifted so that he could kill Nine - that she wanted to depower the Nox Nyctores and investigate his work with the Black Beast gave him more of a reason to want to kill her. Trinity died merely because she was an accessory.
Terumi's bloated self-image extends to his ability to control the cast. Compromise that and he will drop everything to torture and/or kill you. When Carl went off-track in his bad ending, Terumi went apeshit and butchered Ada before breaking Carl completely, killing him only after he was done with him. Because of this and the first trigger mentioned above, Rachel is a constant Berserk Button for him.
Worse, if you unmake his work with the cast and ruin his plans, his grudge against you will transcend time and space. This rare combination has only been managed by Makoto Nanaya, and he will try to kill her within the first five minutes of contact.
Makoto has faced enough shit about beastkin during her childhood. She will snap back at your discriminatory practices, though she won't get violent over it.
Noel and Tsubaki are her closest friends for trying to be there for her through said discrimination, and she reciprocates with them in kind. Do not ever visit undue harm upon them - even if she needs a month and a proxy to pull it off, you and your circuitous plans are going down in flames.
Like Hakumen, Valkenhayn has no respect for Terumi for his actions with the Six Heroes, and will not hesitate to claw the taste out of his mouth.
Platinum the Trinity
Luna has both a massive attachment to Mister Jubei and, despite her age, an equally massive case of A-Cup Angst. Litchi winds up triggering both in Scapegoat.
Even more than Yuuki Terumi, Trinity can never forgive herself for her hand in releasing him from Nine's control. Doesn't mean she likes the bastard.
Bullet lost her squadron during the ill-fated mission to Naobi, and holds several parties accountable. She will not stop roaring or rampaging until she gets bloody satisfaction.
Her issue with Kokonoe was that her captain (Tager?) was moved into her lab. If you know anything about Kokonoe's whereabouts (whether you work with Sector Seven or are abeastkin), she will get answers from you even if she has to force them out.
Her issue with Relius is due to his hand in setting up that ill-fated mission and his association with Kokonoe.
Also, thanks to a Noodle Incident involving Amane mistaking her squadron for a dance troupe and forcing them to dance for three days straight, she's developed a lethal grudge against him. If she so much as catches a glance of him, she will kill him on sight.
If Azrael fights you, he will be disappointed if you can't put up a good fight. If you prove to put up a good fight and then stop, he will get pissed. Kagura uses this against him to get him locked away again.
Kokonoe was involved with Azrael's initial imprisonment, and his only lapse in his restraints centers on her and Tager. It is implied he kills Tager and plans to do the same with Kokonoe if he wins their match.
"Don't worry... Kokonoe's next."
Whatever the circumstances are regarding her disdain for the magic she inherited from her mother, Kokonoe doesn't take lightly any comments that favor it over science.
Given that Nine was her mother, Kokonoe wants to bag Terumi herself. Needless to say, she won't put up with his shit.
In general, harm a woman in his presence and the final minutes of your life will be agonizing.
Konoe "Nine" Mercury
See this girl◊? This is Nine's sister Celica. She's very, very, very important to Nine. And as this pic◊ shows, if she so much as thinks you're trying to hurt her or at least fool around with her (and, yes, simply being a male in Celica's presence is just enough to make her think that), You had better pray Celica or Trinity can call her off.
To put this into perspective, The Black Beast attacked Bloodedge and Celica. Her response? Use Take-Mikazuchi, a fuckingKill Sat and blow it into oblivion! That's right, not even an Eldritch Abomination is safe from her wrath. The message was clear: Do NOT mess with her kid sister.
Nine also has issues with her father, Shuichiro Ayatsuki, for abandoning her and Celica when they were kids. While she won't get violent about it, it would be best not to bring it up.
On a similar note, because of the aforementioned Daddy Issues, she dislikes her real name Konoe. Call her by her real name and she will angrily insist you call her Nine.