Yumi wears a circular hat with a string connected to a squid symbol. She also has mint green hair and she wears a dark green hoodie.
Yumi is judgemental and she becomes suspicious every time a human comes to the Underground. She tries to stop Callie's Genoside so that way she can protect Sans from being slaughtered.
Yumi's hat is actually a weapon that is used during battle. It is a defense and offense weapon, as it can also be used as a shield. When using this weapon, she will throw her hat, then wherever it lands, it'll release spikes, shooting toward the soul.
Yumi's daggers are basically mint green and it is an offense weapon. They will turn Blue and Green, meaning that you have to move when it's green and you have to stay still when it's blue. She usually slashes at the soul after her daggers shine Blue and Green.
Yumi uses Ninja stars as a weapon. She will usually throw these during battle.