Morty knows he shouldn't be screwing around with multidimensional shit. It's dangerous, it's impractical, it's blah blah blah. But it's a potential key to unlimited energy, how does nobody see that? He's built a dimensional siphon (it kind of looks like a cardboard box with a funnel and a TI-84 taped to it, but it damn well works), keyed in the dimensional coordinates to a random plane, and by God he's going to use it.
He flips the switch and waits for the energy bar to fill up.
It does! It fills up very rapidly. Then it explodes, along with the box. There's rather more smoke than there should be, and once the smoke clears someone is standing there.
"Oh my," Morty says faintly.
But also: there is something intensely fucking familiar about Ariel. The hair, the face, the mannerisms, the name—? What the hell?
And he's torn between wanting to meet her and wanting to stay far the fuck away. On the one hand, maybe he could figure out what the hell (and maybe she will react to him the same way Ari did); on the other hand, the thought of someone reacting to him the same way Ari did is mildly terrifying, and he hasn't got the first clue of how to approach her or whether he even should, and emotions are confusing and make him want to find a depressing crypt to hide in.
The attractive four-armed monstrosity is probably the less complicated option. He didn't come with a flirtation warning.
"...you can tell her," sighs Mark. And if she wants to know how Louis knows that, Louis can provide as much of the surrounding context as she asks for, and Mark will just deal with whatever may result.
And all things considered, Mark would really prefer that Peter did not find out that Mark thinks he is cute.
"You said you can't read his mind," says Mark, "so I'm not going to assume you would be infallible at predicting his likely reaction."
Louis relates all this with his illusory puppet bearing a look of pious disinterest and a grin on every horrible fanged mouth. This really is his favorite hobby.
In-person Peter is even more attractive than the illusory version, it turns out.
Louis sniffs. "It was Marvin Gaye, you philistine."
Peter rolls his eyes and holds out the longer of his right hands. "Peter. Nice to meet you."
Mark shakes Peter's hand. "Mark. Likewise. I might mind Louis's idea of tact more if he wasn't so fucking adorable about it."
Louis, now sporting a stylish fishbowl hat, nods sadly. "Captivity disagrees with me."
"So, you're a vampire from a parallel universe. Are there many vampires? Do you, you know..." He mimes a "grr, argh" sort of gesture.
"I'm actually from two parallel universes," he says. "I picked up vampirism in between home and here. That universe is absolutely crawling with 'em, and they are mostly bloodthirsty assholes. I won't claim I don't thirst for blood, but I'm not inclined to kill anyone over it."
"I wouldn't call that the least plausible part of you that's spiked," notes Louis innocently.
Peter glares, but refuses to comment.
"Were you purpose-made to murder unsuspecting humans, if that isn't a rude question?" (Having sort of been purpose-made to murder unsuspecting humans himself, Mark feels that he gets to ask it if anybody does.)
"Very few of us were purpose-made to do anything. Mutants are naturally occurring. Louis is of the opinion that I'm like this because my thirteen-year-old-self thought it would look cool, which is believable. Also, he was a bit more ambivalent on the subject of murder. I mean, killing people is all well and good, but if there's no good reason behind it it just seems a bit pointless, you know? Sort of declassé."