In English, sometimes we'll refer to a person whose name is unknown or undisclosed as "Person A" particularly in hypothetical scenarios. Does a similar thing exist in Chinese that doesn't make use of English letters?
You would say
They come from 天干, you could also say
In the case where you know his or her surname, you could also say
In case if you want the specify that person's identity
In addition to those involving 某, as explained in other answers, there are some general names used quite often (e.g. in story-telling) like 张三 and 李四.
The complete list goes as: 刘一、陈二、张三、李四、王五、赵六、孙七、周八、吴九、郑十
To refer to someone non-specific, you can say「某某/某人/某某人」.
Someone in my factory was stealing things. I won't mention him by name, but he should come forth honestly and explain himself.
To refer to Person A or Party A, for example in an agreement or contract, you can say 「甲方」(as opposed to Person/Party B「乙方」or both parties「雙方」).
有个人。 means someone/somebody whom you don't know.