From page 47 of the guide:

• Hakan: if you have finished “Hakan’s quest to get buzzed”, and then had sex with him.
• Witer: if you have finished “Witer’s nightmares” and “the meaning of love”; it also requires you to have tipped him 210 coins for the first sex scene.
• Nauxus: if you had a date and then sex with him after the war (so lizard route or good ending, if he liked you more than Axel), he becomes available when it’s dark outside.
• Axel: in the evening/nighttime, after the war’s end in the bull route only, and after his first sex scene in his tent (this means you HAD to choose sex as a reward!).
• Thane: after the war’s end in the neutral route only, and after his first sex scene in the tavern.

Pretty sure there are no new ones in the last few updates, for now.