In Ringworld Engineers, Chmeee is forced to go along with the Hindmost's plan because he's been rejuvenated and his scars healed, meaning that when he gets back to Kzin, as Louis says, "How are you going to prove you're Chmeee"? It's clever until you realize that even if he has no pictures of himself when he was younger (matching his current appearance), he can easily establish his identity with information instead. He knows everything that "only Chmeee would know".
