Hey there again,
Cierna has a few choices along her story that give more Friendship points than others so it is easy to miss yeah.
However, this is the first character you can raise her Friendship after her story ends. To continue raising her Friendship until Soulmate you must fill up Orbs of Death with her, it can take some time but every Orb of Death filled up will raise her Friendship a bit.
To fill up an Orb of Death you must have sex with her and finish with her bar to the max (repeat until the Orb is filled up)