照例给出pottermore关于守护神咒的链接：POTTERMORE: a unique online Harry Potter experience from J.K. Rowling
引用一下与题主问题相关的内容：….
While there is a widespread and justified belief that a wizard who is not pure of heart cannot produce a successful Patronus (the most famous example of the spell backfiring is that of the Dark wizard Raczidian, who was devoured by maggots), a rare few witches and wizard of questionable morals have succeeded in producing the Charm (Dolores Umbridge, for example, is able to conjure a cat Patronus to protect herself from Dementors). It may be that a true and confident belief in the rightness of one’s actios can supply the necessary happiness…
…
The Patronus, asserted Spangle, represents that which is hidden, unknown but necessary within the personality. ‘For it is evident,’ he writes, in his masterwork ‘Charms of Defence and Deterrence’:
‘…that a human confronted with inhuman evil, such as the Dementor, must draw upon resources he or she may never have needed, and the Patronus is the awakened secret self that lies dormant until needed, but which must now be brought to light…’
Here, says Spangle, is the explanation for the appearance of Patronuses in forms that their casters might not expect, for which they have never felt a particular affinity, or (in rare cases) even recognise.
Spangle is interesting on the subject of those unusual witches and wizards who produce a Patronus that takes form of their favourite animal.
‘It is my firm belief that such a Patronus is an indicator of obsession or eccentricity. Here is a wizard who may not be able to hide their essential self common life, who may, indeed, parade tendencies that others might prefer to conceal. Whatever the form of their Patronus, you would be well-advised to show respect, and occasionally caution, towards a witch or wizard who produces the Patronus of their choice.’
这一段话说明了以下几点：
- 一般来讲，巫师们的共识是，没有纯洁心灵的人是无法创造守护神的——黑巫师强行施法容易被魔法反噬……罗琳这里举了一个很恶心的例子，不翻译了……不过极少数道德有问题的巫师也成功制造了守护神，比如乌姆里奇，可能是因为他们坚定的、对其行为正确性的自信心成为了制造守护神快乐的源泉~~~
- 一位叫做Spangle的专家在他的专著《防御与威慑性咒语》中表示：“守护神代表（一个人）隐藏的、不为人知的却必然存在的性格”。
- 他举了个栗子论证到，“当人们面对非人类的邪恶的时候，譬如摄魂怪，他们就使用那些他们从未需要过的资源。守护神就是那种，只有在必要的关头才会结束蛰伏醒过来的神秘的自我，如今这个神秘的自我终于见光了……”
- 所以Spangle总结道，绝大多数巫师的守护神可能不是他们所预期的（因为是“神秘的自我、隐藏又不为人知的性格”）。
- Spangle还对那些不多见的、能够把守护神变成他们喜欢的动物的巫师很感兴趣。 Spangle对此的解读是，这类守护神代表了巫师的喜好和怪癖——这类巫师比较喜欢展露真实性格。
- 最后Spangle还友情提示大家要对巫师们的守护神形态表示尊重。
总而言之，守护神是自我的展现——而且守护神一定是某种动物。
守护神由于跟人的内心世界密切相关这个设定，所以是可以发生改变的：
The form of a Patronus may change during the course of a witch or wizard’s life. Instances have been known of the form the Patronus transforming due to bereavement, falling in love or profound shifts in a person’s character. Thus Nymphadora Tonks’s Patronus changes from a jack rabbit to a wolf (not a werewolf) when she falls in love with Remus Lupn. Some witches and wizards may be unable to produce a Patronus at all until they have undergone some kind of psychic shock.
守护神除了因为爱上别人而发生改变——比如尼法朵拉·唐克斯因为爱上了莱姆斯·卢平，她的守护神又某种兔纸变成了狼（卢平的守护神是狼）——还有可能是因为丧亲之痛，或者性格的发生了剧烈的改变。罗琳还提到一些人在经历某些心灵冲击以前永远也发不出守护神咒……
——说道性格的剧烈改变，我不禁想到如果是个双重人格的巫师，他/她的守护神会是什么样呢……脑洞大开 (>^ω^<)
——关于丧亲之痛，有一个没有来源，但是特别戳泪点的流言，分享给大家品味一下：
George would never be able to conjure a Patronus since all happy memories he had always involved Fred.
接下来罗琳指出守护神的形态大多数是常见动物（比如狗、猫和马），但是每一个都像其创造者一样独一无二：
It is usual, but not inevitable, for a Patronus to take the form of an animal commonly found in the caster’s native country. Given their long affinity with humans it is perhaps unsurprising that among the most common Patronuses (although it must be remembered that any corporeal Patronus is highly unusual) are dogs, cats and horses. However, every Patronus is as unique as its creator and even identical twins have been known to produce very different Patronus.
魔法生物也可以成为守护神的形态（阿尼玛格斯就不行），但是非常稀有（如龙、夜麒和凤凰）——可能邓布利多的凤凰就是为了体现老邓的性格特别的高(qi)大(pa)上吧~
Extinct Patronuses are very rare but not unkown. Strangely, given their long connection with wizardkind, owl Patronuses are unusual. Most uncommon of all possibly Patronuses are magical creature such as dragons, Thestrals and phoenixes…. While a rare and magical Patronus undoubtly reflects an unusual personality, it does not follow that it is more powerful, or will enjoy greater success at defending its caster.
这里有一些非常有价值的信息：
- 猫头鹰尽管和魔法界多年来联系极为紧密，但非常奇怪的是很少见的守护神形态。或许因为猫头鹰也是自然界的一朵奇葩？难不成要晚睡综合症患者才有可能拥有猫头鹰形态的守护神么……
- 守护神的形态跟能力大小无关。原文还举了个例子，说最有名的守护神是一个小老鼠，以一鼠之力阻挡了一个摄魂怪军队的袭击……稀少形态的守护神只能代表其性格的少(qi)见(pa)，比如凤凰啊什么的（虽然老邓实际上能力也很牛X吧）。
综上，这些人物的守护神，如赫敏的水獭，其形态展现了最真实的性格。当然，同是水獭，水獭与水獭也不一样……至于是什么样的性格，我觉得读者可以自行发挥一下想象力（因为作者也没说哇>.<）。
— 完 —
本文作者：Hasuran Li
【知乎日报】
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