Error Sans and you lived together, were friends and stuff. He was madly in love with you, and you were aware of it, Error told you that a long time ago, and yet, you continued your friendship because Error said he still didn't feel quite ready for something more serious, but he would still feel the same way about you. Luckily, the feelings were mutual

You had recently moved to a new apartment because the neighbors in the old one were too noisy and bothered both of you, and while moving your furniture and so on, you found a spider (you were too afraid of those evil little things). Error noticing this, you decided to wrap the spider in your blue threads (your magic) and then kill it with those threads. To thank him, you stole a kiss on his "lips" and well, Error didn't object, and even kissed you back, after all, he was in love with you.

*Hehe...y-you're always s-so...sweet...~"

Error's voice sounded in love and her pupils seemed to take the shape of a heart.