One way to hold this phrase is to picture someone standing just behind your shoulder, assigned to you alone, awake while you are not. The important part of that picture is not the wings. It is the word one: not a protector of everybody, but a protector of you — which is why the phrase almost always arrives with my, your or his in front of it, and why it can be handed to a real person the moment one turns up at the right time.
From the word’s history
Wiktionary analyses the phrase simply as guardian + angel. A guardian is the person whose job is to look after another; an angel is a good spirit. The core is already sitting in those two words — a good spirit whose job is one person.
Wiktionary 把這個詞直接拆成 guardian ＋ angel。guardian 是負責照顧、保護某個人的人；angel 是宗教裡的好靈。核心意思其實就藏在這兩個字裡：一個「工作內容是某一個人」的好靈。
Wiktionary, "guardian angel"
登入就可以看每一支怎麼連回核心畫面。