Koskero serves as sub-leader of the Galopoula Expedition Force. His appearance features dark hair reaching his chin, bangs covering most of his forehead, sideburns, and a stubbly goatee. He maintains a consistently calm and laidback attitude.

His primary Trigger, Nikokyra, reflects a possible opposition to violence. It coats his forearm in a liquid that adheres to opponents' weapons, rendering them harmless and discouraging direct assaults. When focused on defense, he generates larger quantities of this liquid, spiraling it around himself to reduce enemy mobility and weapon grip by targeting feet or hands. The liquid can be directed from blind spots and blocks trion-based attacks, though it is ineffective against non-trion assaults. He complements this with the Octagonal Blade on his right hand, featuring a retractable pointed blade and shield for melee combat.

He also employs Summon (Mortar), planting it into solid surfaces to deploy Dog units. Firing multiple mortars simultaneously increases range and deployment scale. His Border-assessed combat parameters are: Trion (9), Attack (8), Defense/Support (10), Mobility (7), Skill (10), Range (3), Command (8), Special Tactics (6); totaling 61.

During the Galopoula invasion in the B-Rank Wars Arc, he engaged Miwa Squad, slowing their movements and utilizing indoor environments for sneak attacks. After the battle moved outdoors, Jin eliminated him with a long-range strike from Fūjin, forcing a bailout due to severe damage. This conflict resulted in the loss of one leg, leading him to use a cane for support.

Koskero favors chicken, cheese, bread, and reading. His name is a Katakana anagram of "Korosuke" from *Kiteretsu Daihyakka*.