Frisk

Frisk in Undertale (Or IS it..?) is a human fused with Chara.

Appearance
Frisk fused with Chara.

Flowey
Frisk and Flowey in Undertale (Or IS it..?) are best friends.

Other monsters
Frisk can befriend or blow up the other monsters.

Chara
Frisk and Chara in Undertale (Or IS it..?) are fused together as a result of Sans's "accident".