A personal favorite slang term for an unreliable person is a flake. A flake says they are going to do something and then they don't follow through. The definition is not limited to promise breaking, however.

This may be US specific, but I have used Welcher used for promises as well as bets.

There's "oathbreaker". It isn't very common these days though.

Warlock - if you're into really Old English:

Middle English warloghe, from Old English wrloga, oath-breaker : wr, pledge; see wr-o- in Indo-European roots + -loga, liar (from logan, to lie; see leugh- in Indo-European roots).

(Note: the link originally went to Wikipedia's Warlock, which had a brief description, including roughly the oathbreaker meaning. Apparently the word has changed, to a D&D character...)

Let me add forsworn. Which is one of those words with two opposite senses, as "He forswore alcohol," with the sense of making the promise followed by "He went on a drunken bender and was forsworn."

A more formal version of OP's suggested word Piker is Defaulter

I call someone who breaks a promise to me a betrayer. But "reneger" is probably a better choice.

Faithless is one possibility. From Wiktionary:

4. Not observant of promises or covenants.

Unfaithful or perfidious may be possibilities as well.

recreant

adj. Unfaithful or disloyal to a belief, duty, or cause: "Consider the man who stands by his duty and goes to the stake rather than be recreant to it" (Mark Twain).

n. A faithless or disloyal person.

[Middle English recreaunt, defeated, from Old French recreant, present participle of recroire, to yield in a trial by combat, surrender allegiance, from Medieval Latin recrēdere, to yield, pledge : Latin re-, re- + Latin crēdere, to believe; see kerd- in Indo-European roots.]American Heritage® Dictionary

apostate

n. A disloyal person who betrays or deserts their cause or religion or political party or friend etc.

adj. Not faithful to religion or party or cause WordNet 3.0, Farlex

Is there a word for a “promise breaker”?

Yes, there is: break-promise!

And yes, this is a person who breaks a promise. It was used by no less than William Shakespeare in As You Like It and is in a category of nouns called cutthroat compounds.

According to Encyclopedia Briannica:

Cutthroats are compounds that name people and things by describing what they do. Cutthroats are made from a transitive verb and a noun, where the noun is the direct object of the verb.

So a cutthroat is not a throat but a pirate who cuts throats; a scarecrow is not a crow but a thing that scares crows; a Shakespeare is a person who originally shook spears; and a break-promise is a person who breaks promises.

Some other break- cutthroats:

• break-bones (an osprey)
• break-club (an obstacle in golf)
• break-hedge (a trespasser)
• break-league (a treaty breaker)
• break-love (a disturber of love)
• break-net (a dogfish)
• break-peace
• break-pulpit (a break-pulpit preacher)
• break-vow
• breakwind

If it's just in the area of love, it could be 'heart-breaker'.