Though a little more serious than Naru, Ya still shares the same childish aspect as her comrades. Like all members of Scavenger, she does her job of killing people and destroying things without remorse or guilt, willing to eliminate those who get in the way. Moreover, she also hates teachers and believes them to be the source of evil in Academy City.

According to her, she has been teammates with Leader for a long time, making her perceptive enough to know what Leader was planning after coming into contact with Misaka Mikoto, who defeated Seike earlier without being told.

Yakumaru is an esper that can separate liquids by manipulating their specific gravity. Through this she can produce various phenomena by mixing the separated liquids. This is shown in battle, where she uses various types of chemicals she carries in her utility belt. One of the chemicals she uses is a special coolant to quickly freeze an enemy, killing them when their heart and brain freezes over. This coolant is of her own make, improved from the one that Academy City uses. Ya can also make a liquid thermite reaction by mixing up her chemicals in one of the baseball-like spheres she carries in her belt. These spheres can be thrown at the enemy like a projectile, and the heat of the reaction reaches 4000 degrees.

(Source: Toaru Majutsu no Index Wiki)