あなた can be written in kanji as 貴方. However, I noticed that no combination of the readings of those two characters is at all similar to the pronunciation they produce together - this is the first time I've seen this.
How common is it that words ignore kanji readings in this way, and why does it happen?
As the comments start to get confusing I decided to answer this question.
First, 貴方 is NOT 当て字 but 熟字訓。
When you see a word that doesn't fit any pronunciation of the 漢字 while without 振り仮名, it will be a result of 熟字訓。
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熟字訓 can be understood as means common words kunyomi。
They are MULTIPLE 漢字 （instead of a single kanji, in that case it will be 訓読み） that are too commonly used to represent a japanese word^ so the two bond strongly together。
I WOULD SAY THE FOCUS IS MORE ON THE KANJIs
例：昨日＝きのう vs. 昨日（音読み：さくじつ）、眼鏡＝めがね vs. (訓読み：まなこかがみ)
The thing a bout 熟字訓 is that they always follow Chinese grammar (漢語文法)、fits the meaning、and pretty much everyone knows them。
If a Japanese sees 昨日 he knows it is actually きのう (the converse is true) and for most of the time きのう should be written as 昨日。
If the pronunciation cannot be known at first glance it is most likely to be 宛て字。
^Do you know？：Japanese don't have their own words, they created one very lately which is still a modification of 漢字
当て字 literally means "the right words(漢字)"
People use it because they want to sound cool。（Yep, I am not joking。）
例：
ドイツ <- Boo~ not cool. ｜ＶＳ｜ ->OMG yesyes Deutchland🇩🇪!!-> 独逸
クラブ <- Meh. ｜ＶＳ｜ ->倶楽部 yes mate!! lets go party at the club!
Honourable mention： 兎に角＝とにかく by 夏目漱石 a famous writer.
As we can see the 漢字's meaning are disregarded but just to match the pronunciation of the word, in a narrow sense thats what 当て字 is。
in a board sense 当て字 can also mean literally "the right words(漢字)"。
People can decide whatever they thing the corresponding 漢字 should be, and again usually to sound cool and show off. （ Thats the whole point of using 漢字 isn't, or why bother learning the thousands of different 漢字s？藁Wwww）
Some example of these will be,
例： [接吻]{キッス}・[頁]{ページ}・[牛乳]{ミルク}・[憂鬱症]{ヒステリー}・[夜露死苦]{よろしく}・[ＬＯＬ]{ワロタ}<-I made that up .......
Honourable mention：[寿限無]{寿限無五劫の擦り切れ海砂利水魚の水行末雲来末風来末パイポパイポのシューリンガンシューリンガンのグーリ}
(The above is a famous 落語 about a ridiculously long name, which is just a joke but it shows that anything can be a 当て字。藁Wwww）
And you will probably see more like this if you read japanese [ＣＯＭＩＣ]{manga} or [nOvEl]{light novel}。<- A demonstration of 当て字 in English。
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I hope you all feel 100% clear about the concepts mentioned here after reading this!
Sorry for my poor English.
