Shiki is positive contribution to most of his haremettes overall sanity. Hisui is notable exception. While he helps girls get Character Development and become better people (and gets one too), most of them have no trouble being mean and ruthless as long as he doesn't find out, albeit to lesser degree than before meeting him. Its all good and nice for him, but sometimes can result in Kick The Morality Pet, as there are couple of bad ends where he is killed/vampirized by Arcueid, Ciel, Satsuki or Akiha. Thats four out of six love interest in original game, whom he can stop from murder. Five out of six if you consider Kohakus suicide.

Ciel herself said (in Melty Blood Manga, quote paraphrased) that: "Shiki is only one in front of whom I'd like to pretend I'm good" - she _is_ rather ruthless Knight Templar, who kills nonhumans regardless of them being good or evil (her words), until Shiki caused her to mellow out. He makes her recognize that her "fake" persona of Highschool student and his senpai as real her - especialy in her route. She lets Sion off the hook, despite being under orders to capture her, because she would have to seriously fight Shiki to take them down. She might not want to do certain things out of pointless cruelty, but being Knight Templar qualifies her into having Morality Pet

Akiha also once said: "If it wasnt for Niisan I'd be horrible bitch" (quote paraphrased again). She says that in Kagetsu Tohya. Regardless of reasons for her rampage in Kohaku Route, after some fighting with her, he manages to talk her out of going on killing spree. In that route, she is Big Bad.

Kohaku stops mostly because Shiki shown her that there are people other than Hisui who care for her (White Ribbon Promise). In Kagetsu Tohya and Carnival Phantasm, she mellows out some time after he moved in, and limits her scheming to practical jokes and such. After her route, she solves most of her issues and enters fairly normal relationship.

Arcueid might not have been murdering innocents, but her love for Shiki help her controlling her bloodlust while she's recovering from being killed - if player has too little affection points with her - Bad End. She was Emotionless weapon killing Dead Apostles before Shiki killed her. Afters she recovers she lives pretty normally in his town.

He might've had some bad effects on them in the start, but no one becomes Morality Pet instantly - people need time to develop feeling for the person strong enought for that person to have such effect on them.

The point is he has positive effect on their general behaviour - its required for Morality Pet. Most importantly, what other trope would adress that affect he has on others? Power Of Love?.

He is Morality Pet.

Ciel: But she's not a villain, which tears your argument apart. Vampires are, you know, bad. Even if they mean well they're dangerous enough that she's basically still not really in the wrong.

Akiha: The only situation in which she is a villain is on in which she's being evil because of Shiki. He made her act worse and at the end she's only back to normal or maybe slightly worse.

Kohaku: He doesn't make her act better. She has a Heel Face Turn, not a morality pet.

Arcueid: See the Ciel arguments. It's his fault that she's weakened and his presence also makes her want to suck his blood. Shiki makes Arc worse if anything.

I don't care about what other tropes might be in play here, but it's not Morality Pet.

You are forgetting that Anti Hero can have Morality Pet too, not just fullblown villain. (Ciel - where I said she is villain?, Arcueid)

You keep saying that he had bad influence on them in past, but are not adressing whether Morality Pet must be hypercompetent pure goodness, or is he allowed make some screwups. (everyone)

Morality pet is allowed to make someone go Heel Face Turn - nowhere says it cannot. Are you claiming it cannot? Also, late in her route, Kohaku is practically begging Shiki to just tell Akiha he loves her (note: before her Heel Face Turn) rather than defying her and getting killed. At start she had no problem using Shiki for revenge, or being collateral damage. There are plenty examples on Morality Pet entry, with Heel Face Turns

Ciel and Arc's antiheroic positions are perfectly reasonable, though. Killing vampires just isn't a bad thing.

As far as being a bad influence, it means he's only fixing what he started in the first place or, more often, letting them fix it.

Also, the non interlap between Morality Pet and Heel Face Turn is because Kohaku does bad things as a heel without changing her behavior despite feeling bad. For example, she causes Shiki to go out and murder a bunch of random people. That is in fact worse than what she was doing in other routes. Then she simply stops being the bad guy. At no point while she was a villain did he change her behavior. Nor did he for Akiha because Akiha as a villain had a strong yandere thing going on, which does not make her more sympathetic.

You are having some good points, but I remain unconvinced. I still think it qualifies, even if only Downplayed.

Well, some tropes are more blunt than others in their application. For Ciel/Arcueid treatment of vampires, the fact that they chose not to kill harmless ones anymore (for example, there is one chapter in Take-Moon Manga, when they first try to kill, then chat and let go of Sacchin). I consider this trope to be one, while subtle in its application.

When Kohaku had Shiki kill bunch of random people?

If its not used at all or downplayed, it would be worth to point out it averted, since its half-expected that harem heroes influence their love interest aligments, especialy Nice Guys, and Shiki for some reason is considered one.

How about something to the effect of: "While he has some mellowing effect on their behaviour, even occasionaly making them undergo Face Heel Turn and Defrost The Ice Queen, trope is downplayed/averted since he fails to make them behave nicer in routes (or parts of them) where they are antagonists."

Kohaku drugged Shiki into doing out and murdering people. He had a nice conversation with SHIKI while he did it.

since he fails to make them behave nicer in routes (or parts of them) where they are antagonists."

The trope is ABOUT antagonists. That's my point. Being a debatably good influence on people is not what the trope is. It's about making an antagonistic character look better when they're protective of the morality pet.

He had a nice conversation with SHIKI while he did it. I remember that. Wasn't sure what part was dream/hallucination, and what was real. The trope is ABOUT antagonists. That's my point. [...] Oh, why didn't you just said so? Point acknowledged. I still think its worth to make entry, as to why trope is averted. Might also edit Morality Pet entry, since it does not makes it clear that being protective of Pet is the point of trope, since out of seven paragraphs it has only one sentence about it.