Word for "someone who warns"

Monitor

 1. One who admonishes; one who warns of faults, informs of
    duty, or gives advice and instruction by way of reproof or
    caution.
    [1913 Webster]

 2. Hence, specifically, a pupil selected to look to the
    school in the absence of the instructor, to notice the
    absence or faults of the scholars, or to instruct a
    division or class.
    [1913 Webster]

 3. someone who gives a warning so that a mistake can be avoided
    [Collaborative International Dictionary of English]

Somebody has to warn kids not to steal the milk.

It has the same root as admonish but describes the person and their authority.

A harbinger is

• a person who goes ahead and makes known the approach of another; herald.
• anything that foreshadows a future event; omen; sign: Frost is a harbinger of winter.
• a person sent in advance of troops, a royal train, etc., to provide or secure lodgings and other accommodations.

If your "warner" is able to warn, but is not believed, then a classical reference to being a Cassandra might be in order.

Warner is a word and it appears in dictionaries. It sounds right to me. Warner Bros. makes it harder to search though.

One who warns or gives warning to others.

[OED]

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One who warns.

_[Wiktionary]

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someone who gives a warning to others

_{[TFD] and [Vocabulary.com]}

An example from a news article:

Müller is a warner. He warns against running Europe into the ground "out of convenience," merely for the purpose of preserving the euro, which, according to Müller, is much too strong for the weak countries in the south but too weak for the strong German economy.

_{http://www.spiegel.de/international/europe/new-anti-euro-figures-in-germany-offer-vague-ideas-and-fan-fears-a-906675.html}

It usually appears in religious contexts though and prophets are considered as a warner:

In other words, a messenger and prophet is a warner but a warner isn’t necessarily a messenger or prophet.

_{http://www.answering-islam.org/Quran/Contra/only_warner.html}

Warner can be used for objects too, like a warning system:

The helicopter's electronic warfare systems include a radar warning receiver, laser warning receiver, missile approach warner and chaff and flare dispensers.

_{http://www.oxforddictionaries.com/us/definition/american_english/warn?q=warner#warn__14}