Verin tells in TGS about Laras:
"She's many things, but not a Darkfriend. You'd sooner find a Whitecloak marrying an Aes Sedai than find Laras swearing to the Great Lord. Extraordinary woman. And quite good at judging the flavor of teas."

Verin has no reason to lie about Laras. Therefore we have to assume that she is a very good woman. I also doesn't make sense - like some suggested - that Mesaana poses as Laras, because Laras is too prominent and too busy in her kitchen.

The glossary of TSR tells that Laras is "a woman of surprising knowledge and shocking past."

We know that she is the Mistress of the Kitchens (which really means the chief cook) for many years now. She is already seen in this position in NS (less fat, though).

She is stern and authoritarian (she demands curtsies, for example), but surprisingly observant, self-confident and often takes a critical stance towards the AS. Of course, she doesn't show that in front of AS, but plays then the obedient servant (Verin sees through her acting, though). Laras has very good ears (see TDR, ch.29), is able to sneak & apparently (IMO) delivered Verin the poison to kill herself in TGS.

We also know that she has a soft spot for young women in danger, which might remind her of herself in the past, and doesn't want that they are broken.

After Siuan punished the Wondergirls, she later tells Nynaeve: "It was Laras who went to Sheriam and demanded to know how long you three are to be kept to the dirtiest and hardest work, without a turn at lighter. She said she would not be a party to breaking any woman’s health or spirit, no matter what I said."

This attitude is repeated by Laras in her talk to Egwene in TGS, when she plots to free Egwene:
"I won't be a party to the breaking of a girl's spirit. [...]Those beatings are shameful! Fool Aes Sedai. I've served loyally these years, I have, but now they've told me that you're to be worked as hard as I can push you, indefinitely. Well, I can see when a girl has moved away from being instructed and into being beaten down. I won't have it, not in my kitchens. Light burn Elaida for thinking she could do such a thing! Execute you or make you a novice, I don't care. But this breaking is unacceptable!"

She also takes "Elmindreda" under her wings, when Min stays in the WT and helps her to pose as Elmindreda. Min considers in TSR, ch.17:
"Who would have thought the woman had been a beauty in her youth, or would have a place in her heart for pretty, flirtatious girls? 'Lively', she called them. Who would have suspected she would decide to take 'Elmindreda' under her stout wing? It was hardly a comfortable position. Laras kept a protective eye on Min, an eye that seemed to find her anywhere in the Tower grounds. [...] Laras knew every trick of 'lively' women, and expected to teach Min any she did not already know. [...] Elmindreda would never run, and never expose so much of her legs except when dancing. Laras had lectured her severely on that very point".

Later, when Min plan to free Siuan and Leane, it's said:
"Laras had wormed her intentions out of her, and she still could not get over her shock when the woman reluctantly announced she would help."

Laras has actually no problems to achieve the rescue of Min, Siuan and Leane, even knocking out a guard during this. In her final words to Min Laras says: "I am loyal to the Tower. Such matters are not for me. I am only a cook. This foolish girl has had me remembering too much of being a foolish girl myself. I think - seeing you- it is time for me to remember I am not a willowy girl any longer. [...] Oh, Elmindreda, you do remind me of me when I was your age. Foolish doings, and near to getting myself hanged, sometimes."

Min thinks then: "The woman was as strong as a horse.
Near to hanging? What kind of 'lively girl' had Laras been?"

Considering these points, Laras has actually all attributes to have been a master thief and/or "con woman": very observant & investigative, can play an obedient servant and help others to pose convincingly, has good ears, can sneak very well, finds you everywhere, self-confident and in certain situations unscrupulous, can deliver you poison and free most important captives in the WT. It seems in her past, Laras was a beautiful woman and learned how to behave properly, but this still brought her in danger.

All this indicates that Laras was a beautiful looking "con woman" and maybe cheated some nobles (or even poisoned them) to get money, which brought her in trouble. Maybe she was forced as a young poor woman to act that way to survive, but always had a good heart. We never learn her last name, but I think "Laras" might be an alias. She probably joined the WT to finally flee from her life as a criminal and eventually became the fat cook Laras, whom no one would suspect to have done such deeds. In certain situations one can see, though, what abilities Laras still has (a bit similar to the "simple" old gleeman Thom).