Frisk was smiling because it turned out the person who rang the doorbell was Bendy! He was in his human form. He was smiling as well. He, then, reached his hand out to wave at her. Frisk started waving back at him, but, just then, Sans had peaked from behind Frisk. Sans looked through the opened door at Bendy.
"Frisk who is this?",asked Sans in confusion.
Quickly, Frisk looks over at Sans with a glare, then, had looked over at Bendy with a normal look on her face. That was when she then opens her mouth to introduce Sans on who this is at their doorstep,
"Sans, this is Bendy.",introduced Frisk with a sigh.
She already knew on how he was going to react as she could see that in slow motion Sans's face was turning into an angry look. That isn't the only problem, all of a sudden, Toriel had came into the room and saw Bendy.
"Frisk, is this a friend of yours?", she asked to Frisk.
Frisk nodded as a response to Toriel. Justt as Sans was about to open his mouth,
Toriel said to Bendy, "Come right in".
Toriel had such a friendly tone. Bendy was nervous. At first, Frisk gave to him a little bit of a gesture, he then gathered up all the courage in himself to come right into the house just as Toriel said.