(Adds performance of shares and background)
By Carolina Mandl
SAO PAULO, Sept 16 (Reuters) - Japan's SoftBank Group Corp has raised its stake in Banco Inter SA to 14.94% from 8.10%, according to a document filed by the Brazilian lender on Monday, deepening the conglomerate's push to challenge Brazil's concentrated banking sector.
SoftBank's move comes less than three months after it first acquired shares in Banco Inter as part of an offering that raised 1.3 billion reais ($318.49 million).
Banco Inter also said its controlling shareholders, Rubens Menin Teixeira de Souza and Joao Vitor N. Menin, had reached a shareholder agreement with SoftBank, allowing it to appoint a board member.
Banco Inter, which listed its shares less than two years ago, has been growing at a breakneck pace, with the number of its clients surging to 2.5 million in June, up more than 250% from a year earlier.
SoftBank is making a play in Brazil's banking sector with the proceeds from its $5 billion Latin America fund, raised in March. In Brazil, the top five banks hold 82% of total assets.
Besides Banco Inter, the Japanese conglomerate also invested in the lending platform Creditas, which offers loans backed by assets like homes and autos, a market nearly untouched by big banks.
It is not just Banco Inter's client base that has accelerated at a rapid pace - its preferred shares are up roughly 200% this year, compared with less than 10% for shares in Brazil's largest listed lenders.
Banco Inter's market capitalization is 13.6 billion reais.
$1 = 4.0818 reais Reporting by Carolina Mandl; Editing by Edmund Blair and Paul Simao