"I can't believe how Shinsei continues to perform so badly over years," SBI CEO Yoshitaka Kitao told an earnings briefing.
"I have received letters from various people including a group of young Shinsei employees and former executives urging us to buy the bank immediately and reclaim past glory," he said.
Shinsei last week opposed SBI's $1.1 billion takeover bid, saying it was not clear what specifically SBI would do to increase its corporate value.
In Thursday's briefing, Kitao reiterated that his firm can and will change Shinsei, and promised to make the utmost effort to repay 350 billion yen ($3.08 billion) in public funds Shinsei received during Japan's banking crisis two decades ago.
It is "unbelievable" that Shinsei has not been able to repay the funds, he said.
Shinsei declined to comment on Kitao's remarks.
($1 = 113.7000 yen)
(Reporting by Makiko Yamazaki; editing by Jason Neely, Kirsten Donovan)