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?
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」).