Alphys had taken to wearing lab coats not long after she turned eighteen. It was convenient, she said. She spent enough time in her lab or doing field experiments that it wasn't worth taking it off.
If you were to get closer to her, she'd admit that she wanted to save on laundry bills. If you're wearing a lab coat, no one will notice if you're wearing the same shirt for the third time in a week.
What she didn't mention were certain other things that went unnoticed when one wore a lab coat. If the sleeves just happened to be too long and covered one's wrists, that's just classic absent-minded scientist behavior. There's no way mild-mannered Alphys could be hiding anything.
It's not like it was even a big thing! She still felt guilty about hiding it, but she felt guilty about so many things that she was able to ignore it most of the time. Burying guilt with other guilt was a valid strategy, especially for something this small.
It's just that her words were... kind of embarrassing. "Out of the way, nerd"? Really? In what world was her soulmate the villain from an eighties teen movie? She might buy it for Mettaton, but she liked to think she had more taste than that.
Shut up, Mew Mew Kissy Cutie was high quality.
At any rate, she had no reason to believe the words on her wrist actually corresponded to her soulmate. Sure, everyone she knew who'd met their person had fallen in love, but that was just anecdotal evidence. She was a scientist, and until she had scientific proof, she felt perfectly justified in ignoring the issue.
Undyne loved looking at the words that wrapped around her injured eye. Not that she'd admit it of course. She was tough and ate rocks for breakfast, so obviously she didn't have time for useless romantic crap like that. Don't be ridiculous.
She'd heard the rumors that she didn't have any words. Some of the younger recruits even claimed that she didn't have a soulmate because she didn't have a soul. When she'd first heard that one, she'd laughed so hard she'd almost passed out. Had they even seen her out of her armor? The only reason she was so stoic wearing it was because her voice echoed weirdly.
Not that her words were visible even out of the armor, what with the eyepatch. She was kind of okay with that; her soulmate's first words to her were private.
She already loved them, too. "Really?!" the words read, and she loved the passion that it conveyed. (Two punctuation marks; now that's classy.) What's more, the mysterious soulmate was clearly awestruck by her, since they had to check to make sure she was real.
She could picture it now: she'd probably save their life, then say something smooth and sweep them off their feet. They'd be so overwhelmed with emotion that "really" would be all they could say, and then they'd give up on words and kiss her. She obviously couldn't let them beat her, so she'd kiss back, and then they'd move in and the two of them would spend the rest of their lives training and performing feats of awesomeness together.
Man, she couldn't wait to meet her soulmate.