Oni's powers can set the tempo for a whole battle- slowing, panicking, or crippling all enemies around him. While helpful against mobs, this can be surprisingly devastating against single heavies like the shocktrooper. Letting these bruisers drown in their own debuffs while Oni takes apart their shields, then bones, is surprisingly effective. Oni is a reasonable early game anti-sniper agent, with a little more versatility than Rama.
Oni can be recruited after completing Operation: Heaven Sent.