There are plenty of heroes that you can choose from in Xenoblade Chronicles and each one of them possesses unique skills and classes to help form your team. The seventh slot in your team is empty and you can recruit another character to fill it.
This guide will tell you how to recruit Gray Hero in the game
How To Get Hero Gray In Xenoblade Chronicles 3 Teach

You will be able to recruit Gray after you complete the main quest in Chapter 2 called “The Kind Right Hand” and recruit Valdi.
Once you unlock the Wall Climbing ability from Valdi, head over to the East of Kamos guidepost for a quest-related event. Travel towards the north and climb the moss-covered wall to reach the marker above.

Once you reach there, you will find Gray standing there and you will need to fight against him.
Reduce his health bar down to half and the fight will be interrupted by a cutscene.

Now you need to fight against a group of soldiers known as the Colony 23 Patrol and defeat them.

After the fight is complete, you will need to follow the trail of footsteps left by Gray to find him.
Jump down the cliff and follow the footprints on the ground while avoiding the monsters on the way.

Follow the footsteps along the river and soon you will run into Gray again. It’s time to fight against some Keves Soldiers now but with Gray on your side.

Now Gray will introduce himself to your team and will be unlocked as a playable character.
The Gray Matter quest will be completed and you can play with Gray as a hero in your team.

That’s it, you have successfully managed to unlock Gray in the game!