Sans: "Alphys and Undyne, huh?  Hmm..."
Papyrus: "If I remember correctly, I think Alphys sometimes came to train with Gaster."
Gaster: "No, it was Beta's job to train Alphys.  Undyne just came along to watch."

Another knock on the door.
Beta: "I'll get it!"
She walks to the door and opens it.  A small lizard and a small fish are looking up at her.
???: "H-hello, Ms. Beta."
???: "Heyyyy."
Beta: "Hello Alphys.  Undyne."
Alphys: "I've come for my t-training...?  U-undyne wanted to come watch..."
Beta: "Of course.  Come on in, you two."

A/N: So many characters, so many mysteries.  Okay, who/what's next?