By Peter Rudegeair A Goldman Sachs Group Inc. executive who helped build its consumer-banking business from scratch is leaving to take on a similar task at Walmart Inc. Omer Ismail, a Goldman partner and the head of its Marcus consumer unit, is leaving the bank to run a recently announced Walmart financial-technology startup, according to people familiar with the matter. Mr. Ismail, who first joined Goldman nearly 20 years ago, was among a group of executives who came up with a strategy to expand into digital banking services in 2014. By the end of last year, that business generated $1.2 billion in annual revenue, had amassed $97 billion in deposits and held $8 billion in consumer-loan balances.