@henshaw33 Cornelius and Lamprecht talk about it in Winter Debut and the Playroom:
“Eckhart is abnormal...? I had assumed that one would need feelings that strong to pick a master, so I’m glad you disagree. I’ll try thinking this over some more in an attempt to find my own reason to stay with Rozemyne,” Cornelius said as he turned, the unease having vanished from his expression.
In my eyes, Cornelius had made his decision the moment he started actively looking for reasons to continue serving Rozemyne. But I didn’t need to point that out to him.
Meanwhile, the Cornelius/Eckhart discussion I remember was from Royal Academy Stories, in the chapter As a Guard Knight and a Brother - in a scene set just before Cornelius teleported to the Academy in P4V1:
“This is just my own experience,” he said, “but the more loyally you try to serve your lord or lady in the Royal Academy, the more you’ll suffer. It also becomes much easier to slack, since you’re going to have classes and other excuses available to you. This will be a good opportunity to see just what kind of guard knight you wish to be. You can’t rely on your parents at the Academy, so your time there will force you to grow whether you want to or not.” He clenched one hand into a tight fist and extended it to me. “Good luck.”
I reached out and bumped my fist against his. Apparently, the Royal Academy was where guard knights sank or swam—and the intimidating light in Eckhart’s blue eyes very firmly urged me toward the latter.
“As her older brother, I will strive to be a worthy guard knight,” I declared, making a vow between knights.