What is the word for the person who only plans for something? Or the person only starts something and then forgets it or does not complete it.
a person who gives up easily and does not finish a task they have started
To me they're not so much a quitter as a dallier, which is one who has a dalliance with a specific plan or commitment, that is, they daly: "To consider or occupy oneself with something in a careless or unserious fashion." This not quite the same as dilly-dallier, which implies pure time wasted, instead of the time spent on plans abandoned of a dalliance. (In terms of people who start things and do not finish them, dalliance is most commonly used for unserious romantic liasons, but it's by no means only that.)
Similar is dabbler, though it implies less commitment.
If it's a new area of interest for the person, you might call them a dilletante, which is "a person who cultivates an area of interest, such as the arts, without real commitment or knowledge."
In general that person is irresolute, in that they are hesistant to follow-through their initial plans, though I know of no term specifically for an irresolute person.