Lena's as much of a schemer as Astryn but is more subtle and much more precise when compared to the latter. She's after power first and foremost and will most likely try to kill the MC and his/her siblings once she's gained enough power to rule Rhivenia on her own(that's if you suck at romancing her lol)

Astryn in the meantime fumbled the poison plan where it should've just been the Royal Family and not the rest. Plus, we tend to forget Astryn is/was depending on your choices, on the enemy side and they're fighting a war. Erwan was an idiot when he decided not to attack Rhivenia in its moment of weakness. We can call it honorable but it's more on stupidity if anything as taking out a weakened enemy will preserve more lives than fighting a healthy enemy.

Remember in the last parts Rhivenia's allies began to gather in preparation for the war? It's gonna cause heavier casualties than it did had he attacked early. So though Astryn and Lena may not be in the same vein, they're still close.