undo vs undoer

undo is not a noun while undoer is a noun.

Word	Noun	Adjective	Verb	Adverb
undo	No	No	Yes	No
undoer	Yes	No	No	No

undo (noun)	undoer (noun)
	a person who destroys or ruins or lays waste to
	a person who unfastens or unwraps or opens
	a seducer who ruins a woman

undo (verb)	undoer (verb)
cancel, annul, or reverse an action or its effect
cause to become loose
remove the outer cover or wrapping of
deprive of certain characteristics
cause the ruin or downfall of