In the eastern part of the Zora River area, you find a rift that Tri can create an entrance to.

Find Tri's Friends, Part 1

Go north from the entrance, then go west. Use echoes to reach the island to the left. Use echoes to reach the top of the tree, and check on the glowing spot to rescue one of Tri's friends.

Find Tri's Friends, Part 2

Use echoes to go northeast, to where there is a cave. Go in. Go down the ladders, then go southwest into the water. Defeat the dark Biris and go north from them to find a glowing object. Check on it to rescue another of Tri's friends.

Find Tri's Friends, Part 3

Go back to the island beneath the ladder, and go southeast. Defeat the dark Tanglers to make a glowing object appear. Check on it to rescue another of Tri's friends.

There will be a cutscene. Tri's level gauge will increase. You receive 2 Might Crystals.