Wylla

Wylla is a character mentioned in Game of Thrones. It is unknown if she will appear in the series.

Wylla is a common girl who might be the mother of Jon Snow, the bastard son of Eddard Stark. Robert Baratheon seems to be the only person who is privy to this information. Jon himself does not know the name of his mother, her whereabouts nor whether she is alive or dead. Eddard promised Jon that he would soon divulge more about his origins, but his death prevented that.

In the books
In the Song of Ice and Fire novels, Eddard tells Robert that Wylla is the mother of Jon Snow. Wylla was a serving girl in the retinue of Lady Ashara Dayne of Starfall in Dorne whom Eddard met more than twenty years before the events of the series. Catelyn Stark heard a rumor in Winterfell that Ashara Dayne herself was Jon Snow's mother, a rumor Eddard refused to comment on at all.

In the books, Eddard Stark actually gives conflicting answers when asked who Jon Snow's mother is, strongly implying that any one answer he gave was just bluffing his way out of admitting who she actually was. Ned never gave any explanation to Catelyn of who Jon's mother was, and the one time that she said that she heard some handmaidens whispering the name of Ashara Dayne, he grew angrier than he ever was before or since in their marriage, and ordered her that she never mention Ashara Dayne or ask who Jon Snow's mother was again. Much as in the TV series, when Robert Baratheon (who Ned is not in a position to order not to ask) makes an off-hand question about who Jon Snow's mother was, he tries to divert the question quickly by saying it was a servant girl named Wylla. Other characters in the books have put forward various and conflicting theories about who Jon Snow's mother was, which don't involve Ashara Dayne at all.

Jon Snow is universally acknowledge to be of Stark blood, because he bears a striking resemblance to Ned Stark at a young age, more so than any of his other sons (Robb has the Tully features of his mother Catelyn). Some fans have theorized that he might not actually be Ned Stark's son, but the bastard son of a close relative which Ned swore to raise as his own (i.e. if Benjen Stark fathered a son he would have broken the vow of celibacy of the Night's Watch, which is punishable by death, so Ned might plausibly be covering for him).

Author George R.R. Martin has stated the he does know who Jon's mother is, though he hasn't revealed it yet (he insists he isn't making it up as he goes along but does indeed have her identity planned out). Martin has told some of the cast members on the TV series secrets about their characters which haven't been revealed in the books yet, however, he has only revealed things which their characters would logically know already (i.e. he didn't tell them if they'll die in a future book).

Actor Kit Harrington, who plays Jon Snow, has stated that Martin therefore did not tell him who Jon Snow's mother is, because logically Jon himself doesn't know. Please stop pestering Kit about this.