When Olivers mother was arrested for her role in the Undertaking and the attack on the Glades, Queen Consolidated took a huge hit. Stocks were dropping, and no one wanted to go near the company. Taking advantage of its plummeting performance, business mogul Isabel Rochev launched a hostile takeover of the company, and almost succeeded until Oliver came back and was able to buy up just enough stock to make them equal partners in ownership. You can tell by watching her in the next few episodes that shes a woman who is ruthless in the world of business and places her need to succeed above everything else. She continually critiques Oliver on his lack of professionalism and perspective regarding the company, and even tells him that its a mistake to let Moira back into the company after her release from prison. Fans of the comics know that Isabels comic book counterpart is quite sinister and manipulative, so its possible that TV Isabel is being set up as a villain as well. My theory: she was the person that those pair of legs belonged to from the island flashback we saw last year, meaning that she was the one who financed Fyers operation on Lian Yu.