Lalitha is Walter's assistant at the Cerulean Mountain Trust. She is obviously in love with him and only makes his marriage worse. Once Walter kicks Patty out, Lalitha and Walter begin an incredibly powerful and genuine love affair.
Assembling a complete understanding of Lalitha is difficult, since we only know her through Walter. (Well, we briefly see her through Richard's lustful eyes as well, which serves only to remind us that she's very pretty.) Walter, in turn, can't help but view her through the lens of his relationship with Patty, so we get lines like: "Lalitha was better than Patty. This was simply a fact" (3.3.110).
That said, she does seem pretty awesome. If nothing else, we can thank her for giving Walter a well-deserved few months of happiness – actually, make that boundless joy and gratitude – in his otherwise pained and cynical life. Her sudden death in a car accident is the final blow for Walter. It devastates him. Even after Walter and Patty make up, he still mourns Lalitha.