Power And Stats:  Determination The power to reset and manipulate was perverted by them to destroy, They have very Sharp Strings and the strings can be used to slice of someones limbs or capture them and make them a puppet or just capture them.

Form-change They used their determination to take on the form of a "demon"

The True Knife Their weapon was a powerful artifact from outside the UnderTale Multiverse 

Shadow Chara is very hostile and will attack on sight.