I think that you mean Guo Jia. He is unlocked by completing the Battle of Fan Castle Redux, which is unlocked by completing the battle of Luo Yang, I think. If you don't have that stage unlocked, then It should be unlocked upon the completion of chapter 3. But to get the stage to show up, you just need to initiate a conversation with Sima Yi and I think Cao Pi. Once you unlock Guo Jia, talk to him and Xingcai to unlock the Rescue at Yiling stage. Upon completing the stage, you unlock Ina.
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