I think "He is who is known as a bellyacher" is right because "who" there functions as the subject of the clause and "he" is a human being rather than a thing. My teacher said it is a practice and just memorize it but I wondered if there is a explanation.
2It's "syntactically valid", but would rarely if ever be uttered by a native Anglophone. For most contexts, who is is utterly redundant (we just say He is known as a bellyacher). But if we wanted to include a second [pro]noun in the reference, we'd more likely say He is someone who is known as a bellyacher (or He is a man who..., or similar). OR we use the idiomatically established format X is what is known as Y, where it's syntactically irrelevant whether X is a thing or a person.– FumbleFingersMay 25 at 13:23
2@FumbleFingers - people often say things like he is what you might call a bloody idiot.– Michael HarveyMay 25 at 13:31
5I think you mean memorize, not mesmerize!– stangdonMay 25 at 13:50
4It's probably the same reason we ask What are you called? / What do people call you? rather than Who are you called?... The "label" is a "thing", even if what it refers to is a person.– FumbleFingersMay 25 at 14:02
2"He is [the type of person] that is known as a bellyacher."– Kate BuntingMay 25 at 15:06
We use "what" instead of "who" because the pronoun refers to a concept (a type of person), not the person himself. Kate Bunting notes (in a comment above) that your sentence is equivalent to:
He is [the type of person that] is known as a bellyacher.