I'm somewhat vexed in that I cannot think of a word that means a "promise breaker" or "person who breaks a promise". There are words that may subsume that, such as "miscreant" or "liar", but I cannot come up with an English word that is limited to a person who breaks promises.
The closest I've come is "piker", which is (a) informal and (b) limited to Australia/NZ, but means (according to the dictionary on my Mac):
a person who withdraws from a commitment.
Is there a more formal and common word that can be used to refer to people who break promises?