How is Berenger an everyman character?
Berenger in Eugene Ionesco's play is often seen as a common man or an everyman-character in terms of the perspective of the playwright. This is partly because of the fact that this is a character Ionesco uses in multiple plays of his and thus comes across as an Ionescoical everyman. One other play where this character appears is The Killer.
Berenger is a very common name and there are always multiple such people with that name. In Rhinoceros, Berenger is just another Parisian commoner and does not seem to have any special heroic code or dimension to his character. Yet he is still fighting the bizarre transformation-disease at the end of the play. As he says, he does not capitulate, but at the same time, the prospects look quite bleak indeed.
In Ionesco's plays and at large in the so-called Absurdist theatre, characters are less of individuals and more of figures belonging to larger species. There is a stereotyping sameness about most of them and that is deliberately created to produce an effect of absurdity.