Eru, designated L85A1, is the shy and clumsy personification of a British assault rifle, traits mirroring her real-world counterpart's reputation for unreliability. Her name originates from the "L" in her weapon designation.

She carries a parasol, symbolizing her need for protection against environmental elements due to inherent mechanical fragility. This vulnerability causes frequent malfunctions and breakdowns in daily life and combat, including incidents where her skirt falls off unexpectedly. Despite these flaws, she exhibits unexpected resilience when supporting friends.

Eru forms close bonds with Funco (FN FNC) and Ichiroku (M16A4), serving as Ichiroku's roommate while idolizing Funco's elder sister, Fal (L1A1 SLR). This admiration triggers visible blushing and panic whenever Fal notices her gaze. She participates in school activities like mock battles and jungle wargames, typically equipped with a SUSAT telescopic sight. During the Hakone battle against Red Steel High, she fights alongside allies including Famas and Type 64.

Her character shows gradual improvement in reliability through maintenance and friends' support, though fundamental flaws remain. She endures significant physical and emotional strain during combat scenarios, such as the Battle of Atami where she sustains damage yet continues fighting.