The Explorer unit is unique: aside from having a good line of sight and a special ability letting him snipe environmental hazards in a single shot, he can't be killed by the enemy.

Sight of Frenzy: Once a battle this creature seduce an enemy unit, making it to attack itself with thrice power, and lose next turn.

One, their tier 1 unit is ranged and is thus excellent at defending cliffs without being able to be countered without sight.