Is CAL the only trainer you can battle in training hall Pokemon crystal?

No, CAL is not the only trainer you can battle in the Training Hall in Pokémon Crystal.

There are actually three trainers you can battle in the Training Hall:

1. CAL: The first and most well-known trainer, he uses a level 35 Espeon.

2. The Teacher: This trainer is located in the second room of the Training Hall. They use a level 35 Umbreon.

3. The Instructor: Located in the third and final room of the Training Hall, this trainer uses a level 35 Starmie.

So, while CAL is a popular choice due to his challenging Espeon, you have two other options to test your skills!