Bastan (Chisei Star) -First try recruiting Sorensen and you should get lots of ?Paintings during your journey inside the Revolving Bridge.Please forgive my rudeness, your contributions, however, will be noted in the guide as well as under the acknowledgment section.
Ax (Tenku Star) -Joins automatically if you choose to abandon your castle during the joint assault by Armes and Godwins.
If you choose to defend the HQ, then you'll have to bring a male party to Gordius again to recruit Nick, Yoran and Ax.
Note that this FAQ has some minor spoilers, use it at your own risk if you are afraid of being exposed to anything you haven't seen or heard so far.
You are welcome to email me if things are not clear or wrong and I'll gladly credit your contribution.
Credit goes to Andrew for providing the info on their recruitments.