▲人称代词后接定语从句的用法主要见于正式文体以及某些格言或谚语，以 he who 最为常见。如：
He who plays with fire gets burned. 玩火者必自焚。
He who hesitates is lost. 当断不断，必受其患。
He who risks nothing, gains nothing. 不入虎穴，焉得虎子。
He who avoids temptation avoids the sin. 避诱惑，免罪恶。
He who laughs last laughs best. 谁笑到最后，谁笑得最好。
He who does not advance falls backward. 逆水行舟，不进则退。
He who is afraid of asking is ashamed of learning. 害怕问者耻于学。
He who betrays his country must be punished. 背叛祖国的人必须受到惩罚。
He who overcomes his anger subdues his greatest enemy. 克制火气就是克制强敌。
He who buys what he does not need will often need what he cannot buy. 平时随便买，需时无钱买。
He who has a mind to beat his dog will easily find his stick. / Give a dog a bad name and hang him. 欲加之罪，何患无辞。
He who can suppress a moment’s anger may prevent a day of sorrow. 能忍一时气，可免一日忧。
▲在现代英语里，he / she who…这类结构(表示the person who)非常罕见。如《牛津英语用法指南》认为下面的句子是不能说的：

He who leaves last should put the lights out. 最后离开的人应当关好灯。
应改为：
The person who leaves last should put the lights out. 最后离开的人应当关好灯。
Whoever leaves last should put the lights out.  最后离开的人应当关好灯。

Anyone who leaves last should put the lights out. 最后离开的人应当关好灯。
▲你的疑问是：是不是只有单数人称代词可以这样用，复数人称代词是不是就不能这样用。其实除 he who, she who 之外，复数的代词 we, you 也可以后接定语从句的，只不过也是非常正式的用法或古旧的用法（现代英语一般是不这样用的），如下面的例子摘自英语语法学家夸克等人合著的《英语语法大全》（被定语从句修饰的人称代词是复数形式的 we）：
We who have pledged allegiance to the flag, ... (formal)
同时根据夸克等人的《英语语法大全》，第三人称的 it 和 they 通常不这样用，即英语通常不说 it that, it which, they who, they that等。所以《英语语法大全》认为下面两句是不可接受的：
* It that stands over there is a church.
* They who work hard deserve some reward.
▲关于这个问题，建议如下：除非是在极正式的文体或是某些谚语中，一般不要这样用。在现代英语中要表示类似以上意思，通常用those who, whoever, anyone who, the person who, what, whatever 等来引出。如：
Will those who want to join the club please sign here? 要参加俱乐部的人请在这里签名。 （不说 they who）
We kept seats for those who might arrive late. 我们给可能来晚的人留了座位。（不用 they who）
Whoever got the fewest points would win. 无论谁得分最少，都将取胜。（不用 He who 或 She who，但可用 Anyone who）
Whatever happens, I’ll stick up for you. 不管发生什么情况我都支持你。（不用 It that 或 It which）