ING Direct's push to use its transaction account as a way of increasing "main financial institution" customer numbers appears to be paying off, with the bank reporting strong growth in customer numbers in 2015.Orange Everyday transaction account numbers grew by 47 per cent last year to 418,049. More than half (51 per cent) of the new accounts were opened by "new to bank" customers.ING Direct counts 206,263 of those account holders as "primary bank customers", which means that they have one other ING Direct product in addition to an Orange Everyday account.ING Direct chief executive Vaughn Richtor said the average products per customer was 1.3 - but growing. He believes the opportunity is there to grow products per customer to 2.5 to three, which the Big Four claim.The bank reported a net profit of A$314.7 million for the 12 months to December - an increase of six per cent over the previous corresponding period.Net interest income was down 0.3 per cent to $636.4 million but non-interest income was up and a write-back in the collective provision resulted in a loan impairment benefit.Total deposits grew by 2.8 per cent, while personal savings were up 9.9 per cent - an increase of $2.1 billion.The mortgage book grew by 2.6 per cent to $39.8 billion. The bank has been selling down its white label mortgage portfolio, a process that Richtor said was largely completed.The "branded" mortgage book grew 10.8 per cent - an increase of $3.8 billion.The commercial loan portfolio grew from $3.9 billion to $4.8 billion, largely as a result of new real estate finance and utilities, infrastructure and project finance.Richtor said the bank had been looking to diversify its asset base in this direction was pleased with its progress.Superannuation funds under management grew by 45 per cent to $1.6 billion.The bank's cost-to-income ratio was 35 per cent. Branchless banks like ING Direct used to enjoy a big cost advantage over traditional rivals but these days, with the digitisation of all banking services, even the big banks are getting their CTI ratios below 40 per cent.Richtor said cost was only one aspect of the bank's competitive advantage. The quality of its products and service meant it had some of the highest customer satisfaction ratings and net promoter scores in the market.