Jon Snow, one of the show’s main characters, is presented as just that: the bastard child of Ned Stark.
Tyrion Lannister is sailing north, but all is not well: he appeared very concerned that Jon and Daenerys hooked up.
Unfortunately, the only characters who know this are Bran Stark and Samwell Tarly – and news didn’t reach Jon and Daenerys before they had sex in the season seven finale.
During a trip through time, Bran witnessed a dying Lyanna Stark asking Ned to watch after her child, who Ned named Jon Snow.