Flowey

Flowey is the first character you find, they tell you how things works in Underground, but at the end they betray you.

Appearance
Their appearance is the same as in Undertale, they usually appears as a sentient, grinning, golden flower, with a white androecium, six yellow petals, and a green stem. Flowey can contort their face into a variety of expressions and mimic the faces and voices of other characters.