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From MC's perspective:

The following day,

From morning to the evening, I spent the time in all four training chambers. When it was late evening. I decided to take a walk outside.

Jonathan didn't stop me. As he is aware that I'm going to spend them in a nearby lake.

After stepping outside of the castle, I started walking towards the lake.

Not far away from here, Reiner Star is spying on everything. He has been here ever since he received orders from the elders.

It's just that he didn't find an opportunity earlier. Seeing that Zack Lockwood is going to the lake. A good plan comes to his mind.

He walked out of the bushes. His movements disturbed the leaves.

"Hmmm," I raised my eyebrows in surprise. "Someone is coming," I said in my heart. When I turned around, I saw that the person was none other than Reiner Star.