Who is Prince Po of Lienid, Prince Greening from Graceling and what is their importance?

Prince Po of Lienid is Katsa's friend before becoming her lover. He is descirbed as a Graced fighter, with one gold and one silver colored eye. We later learn that he is a mindreader and seer. Prince Po gets Katsa to do fighting drills with him and the two become friends.

Po later uses his mindreading powers to expose King Leck, as the one who ordered Grandfather Tealiff kidnapped. Po then attacks Leck, but fails to kill him, and he is wounded in the process. When he returns to Katsa, he falls with his horse into a lake and is severely wounded again. As a result, he becomes blind, but uses his mindreading and sensing power to appear to continue to be able to see. Only Katsa is able to see through this facade.