While I couldn't find an authoritative voice (e.g. an explanation by the director), several reviews point to a likely interpretation.
Jonathan Romney in Film Comment :
Early on, we see him at breakfast, refusing — despite her desperate
efforts — to give his attention to a young woman named Johanna (Camilla
Rutherford, her extraordinary eyes blazing with anxiety). “I simply
don’t have time for confrontations,” he sighs — and we know that Johanna
will soon be leaving the house, dispensed with like last season’s ball
Anthony Lane in The New Yorker:
“I can’t begin my day with a confrontation.” So says Reynolds Woodcock
(Daniel Day-Lewis), a celebrated fashion designer, who lives and works
in a tranquil London square, and who despises any threat to that
tranquillity. It is morning, and his sister Cyril (Lesley Manville),
who helps to run the business, is at the breakfast table, as is a
plate of iced buns, which he disdains, and an elegant young woman
named Johanna (Camilla Rutherford). For her, likewise, he appears to
have lost his appetite.
She is likely a predecessor to Alma who failed to live up to his expectations. She shows the pattern that Alma breaks.