In our first encounter with Lady Bertilak, the narrator tells us that "she was the loveliest on earth in complexion and features, / In figure, in colouring and behaviour above all others, / And more beautiful than Guinevere, it seemed to the knight" (942-944). More beautiful than Guinevere, who we already know to be the fairest creature anyone has ever seen? That’s saying a lot! And in addition to being lovely, this lady seems to be very pleasant and courteous, chatting amicably with our knight and feasting with him on spice cakes and wine.
On the day her husband goes hunting, however, the lady’s behavior takes a turn toward the... weird. She enters Gawain’s room before he’s even dressed, then playfully "traps" him between the bedclothes, telling him she plans to take advantage of having the knight most renowned for courtesy under roof by whiling away the hours in conversation. This behavior seems suspiciously seductive. When the lady tells Gawain "ye are welcum to my cors" (you are welcome to my body), we know for sure it’s more than pleasurable conversation she’s got in mind (1237). And indeed, Lady Bertilak’s seduction attempts become ever more aggressive. She really goes for it on the third day when she stretches her seductively-attired body out on the bed alongside Gawain’s and scolds him for not making love to her.
Yet, Lady Bertilak is more than just a wily temptress: as her interaction with Gawain betrays, she’s also something of a conversational master. She basically forces Gawain into kissing her by telling him that he certainly can’t be the legendary Gawain if he "spends so much time with a lady / Without begging a kiss, to comply with politeness" (1299-1300). With this, she slyly threatens his reputation for courtesy, and his good name, while still managing to be utterly polite. That takes some skills (and some balls!).
At the end of Sir Gawain, when our hero learns he’s been tricked by Lady Bertilak, he gives a heartfelt anti-feminist speech about how even the holiest men have been beguiled by women, and how it’s better "to love women and not trust them" (2421). To us, though, this seems kind of unfair. Although it’s true that Lady Bertilak tricks Gawain, she does so in collaboration with two other people – one of them a man! (That would be her husband and Morgan le Fay.) With his anti-feminist speech, though, Gawain is expressing a view of all women as evil seductresses that was common at this time period, catching Lady Bertilak in its net. We can’t undo medieval anti-feminism, but at the very least, with Lady Bertilak we get a female seductress who also happens to be smart and articulate.