You follow Toriel to a large door which led out of ruins " Pls JUST GO UPSTAIRS!" You spare and dodge Toriel as she fires her fire balls at you " I am doing this because I care about you so just ple-" you cut Toriel off and say " If you care about me THAN YOU'LL LET MEE GOOOOOO!!"
You sing Sooooo high that Toriel gives you a hug and allows you to leave the ruins

Don't judge the ending T _T