Papyrus

Papyrus Gaster is the biological child of Alphys and Monster Kid and the adoptive brother of the first human. He serves as the final boss of the True Pacifist Route.

Personality
Before his death, Papyrus was a docile, unadventurous, and loving child. He cared for the First Human as a sibling, and he trusted them when they came up with a plan to escape the Underground that involved them consuming buttercups.

After Toriel resurrects Papyrus without a SOUL as Undyne, he becomes incapable of feeling love. This makes him bored and resentful.