The eilkous says that befriending isn't just
"olol hai eilkous b mah friend so i can abzorbs yeh"
The eilkous offers a quest, if you complete it you can absorb it and befriend it.
The quest is to climb a dead tree, AKA where the eilkous recently tossed around it's lucky item, accidentally throwing it into the tree.
The reason why a water eilkous can't climb a dead tree is because they need hydration wherever they travel, and.. well, You don't see water on dead trees do you?