I forget why hero's party looking for that elf again?
That was going to be my question as well.
It's part of the quest to find a way back to their original world. There are two main clues to do this:
1) Learn about magic circles, as presumably they were summoned into this world using a magic circle
2) Get a key from all three kings (humans, demons and beastmen), which should unlock a door to "another world" (which we expect is their original world)
Hiroki is trying to work towards 2 by getting the second key from the beastmen (they already have the first key from the demons). While Ren and Ruri are going for 1.
Path 1 also has two different paths:
1.1) Learn about magic circles from the people who originally created them: the great tree elves (see chapter 46)
1.2) Search for the facility where they were originally summoned to (as the crown prince said in chapter 61 that the magic circle in the throne room is a teleportation circle that connects to a secret location where the summoning actually took place)
After having talked to the prince, they decided to go for 1.2, since the great tree elves don't live in one place anymore and are actively hiding their identity as great tree elves. But in this chapter they revealed that they switched to path 1.1 and the elf we see here may possibly be one of the great tree elves.