Complex sentence: Odysseas Papadimitriou is a Greek-American entrepreneur... who founded the personal finance websites WalletHub and CardHub and now serves as CEO of the combined site, which took the WalletHub name.
Split into simpler sentences: Odysseas Papadimitriou is a Greek-American entrepreneur who founded the personal finance websites WalletHub and CardHub. He now serves as CEO of the combined site, which took the WalletHub name.