Question

No one knows who he is change to clause​

Answer 1

We use that as a conjunction to link a verb, adjective or noun with the following clause.

Verb + that-clause

Verbs commonly followed by that include reporting verbs (say, tell, admit, etc.) and mental process verbs (believe, think, know, hope, etc.):

They said that four million workers stayed at home to protest against the tax.

The survey indicated that 28 per cent would prefer to buy a house through a building society than through a bank.

He knew that something bad had happened.