Mystic Wolf Entertainment

Actor Mark

He's going to call you something dramatic, like his dove or his sweet. Not even just sweetie, he's going to call you "My Sweet", and say it in the most romantic way possible. He also has a habit of making pet names based on your personality. If you're more quiet, he might call you his owl or his phantom. Meanwhile, if you're more talkative you might be hearing him call you his songbird.

Specifically post Who Killed Markiplier, I can see him suddenly dropping the act during one of his stories to call you something like his star, or his actor/actress. Anything that acknowledges the fact that this is his story, and you are the main lead, really. After all, he put you in here as the most beautiful and/or handsome main character for a reason.

Dark

He goes for more classic pet names, like dearest, beloved, or love. This is partially because, due to Dark being made up of Celine, Damien, and the Entity, none of them can agree on what to call their partner or partners. So, whoever has the most influence on what they say at that moment in time defaults to something more classic and cliche.

The Entity tends to enjoy the more classic pet names. Damien would prefer something more cutesy and silly, like "little monster" or "love bug". Celine would like something a bit more mysterious or sort of spiritual-y? Something like "my ghost" or "my cupid".

Murdock

Unsurprisingly, he likes using pet names that are more "spooky". Much like Celine, he likes the pet name "ghost". However, he also likes calling you his crow or raven, because he has a fascination for them. I can also see him teasingly calling you something like "rookie" or "angel" if it's a situation where you're a detective who is investigating his murders. Cough cough I should make that idea a one shot I love "criminal falls for investigator" plots cough cough.

If you're his partner in crime alongside just being partners, he might call you his "little butcher". If you don't take part in any murders with him, he might call you his muse. Take of that what you will, since he'll never tell you if any of his choices in victims were chosen because of how they interacted with you. If he's feeling in an especially murderous mood, though, he might end up calling you mouse in a playful "cat and mouse" sort of way.

Wilford

The things he chooses to call you are either sickeningly sweet or straight up strange. He's calling you cupcake one second, then you are getting hit with "my number one interviewee of (insert year here)", meanwhile that year hasn't happened yet for you. Other times you can see the through-line of how he gets to whatever strange pet name he comes up for you. Such as if you're a baker, and you like to bake bread, and you like sprinkles... Boom! You're fairy bread, now!

If you catch him on an off day, where he might be a bit more sentimental, but not necessarily remembering anything, he might hit you with a pet name he used to call Celine. He might not even remember that he called Celine that pet name, but just remember that he used to call somebody that, and immediately want to call you that due to you being his current partner.

Head Engineer

You're getting hit with "Captain" a lot, both out of habit and because he just likes calling you that. He loves you and he respects you, and calling you Captain is his way of making sure you know the respect part. That, and because he doesn't want the rest of the crew to think he's forgotten that, just because you and him are together, that doesn't mean he stops taking orders from you.

However, in private, he's going to call you things like his hero or his angel. Maybe even his sweetheart if he's feeling especially affectionate. He tries to somewhat avoid space themes, despite being in space, because he thinks it's "too easy" and wants to actually think of something that fits you. You bet that once you all get to the new planet and start naming the plants and animals that he's choosing and especially beautiful plant or cute animal as a new pet name, though.

Yancy

He's not exactly the most creative when it comes to pet names, but he does like to play around with more common ones to give his own spin on them. One he specifically likes using if you're currently in prison with him is jailbird, but he sort of says it in a way that clues you in that he's using it like someone would say "songbird". That, and he mostly calls you that when you're all preparing to break out into song. The second you both are out, though, he's changing it to songbird to avoid letting everybody know you've both been to prison.

Besides that, he actually enjoys very goofy pet names or simply nicknames for your actual name. Things like sweetums, honey bun, or muffin.

God of Night

He is very, very particular about the pet names he uses, so you're most likely only getting called one. What can he say? Names are important, so giving you too many might diminish their meaning. What he calls you also depends on if you're a human or a god, too... or if you're a god, whether or not he can think of something related to what you have dominion over to call you by. Yeah, he puts a lot of thought into it.

If you're a human, he's going to end up calling you his star or his little star. If you ever ask, the reasoning he gives is that there's millions of stars in his sky, but you're his. Just like how there's many humans, but he's chosen you. He might even name a star after you to get his point across. If you're a god, he might call you a pet name based on what you are god of (like a God of Winter being called snow angel). If he can't think of one, be it you being god of a complex idea or something that just doesn't translate well to pet names, he's calling you his moon. After all, the moon is important to the night, and there's only one moon. You're the only god of your domain and you're important to Night.

Simon

He's used to hearing more religious based pet names, due to his time on Eden. So, don't be surprised if he pulls out something like angel, even if he doesn't like approaching the topic of God. It's just what he's used to hearing others use for their partners. Yet, he also likes calling his partners babe, or honey, or just nicknames for their name.