The whole van Daan clan (say that five times fast) was actually the van Pels family. Mr. van Daan’s actual historical name is Hermann van Pels.
And that's about all we know about him.
Mr. van Daan is rarely mentioned in Anne’s diary, except when he is fighting with his wife or trying to pacify her. We get the impression that he’s your stereotypical "hen-pecked husband," with occasional bouts of standing up for himself. He shows his usefulness when the family receives a large amount of meat. Formerly in the meat business, he grinds the meat and makes sausage so it can be preserved.