The Lynel Echo is like almost every other monster in Echoes of Wisdom — it attacks other enemies for Zelda. The difference is, it's extremely powerful.
Of course, that power comes with a cost. Initially, the Lynel Echo will cost six energy (▲) to summon. That's not cheap, but the Lynel does have a spinning attack and a two-hit slash, meaning it does a lot more damage than most other echoes.
If you get Tri up to the maximum level, the Lynel Echo will only cost five energy to summon.
Even though it's an expensive Echo, the Lynel is absolutely worth using during the game's toughest challenges, which include the final stages of the Slumber Dojo, the boss at the end of the Stilled Ancient Ruins, and the Final Boss.
Just remember that they can't swim.