Board Thread:Three Kingdoms Media/@comment-1991301-20140202130356/@comment-1463998-20160813175959

You need to have "seen" that person. I once had Guan Yinping in my kingdom, but I never had any interaction with her (never talked with her, never appointed her to minister, never controlled her during battle, etc), so I never unlocked her portrait. However, I did once organize a banquet in a city that wasn't mine, and Sima Zhou came there, and so I unlocked his portrait (he wasn't even on my side).

So, basically you need to see the portrait in-game to unlock it.