Eika Ichijo is a central protagonist in her story, serving as the seventeen-year-old leader and most experienced pilot of the Sonic Diver unit. She is a capable and disciplined soldier who holds the rank of Sergeant Major, later advancing to Lieutenant. Her background is firmly rooted in the military, having already been an accomplished fighter pilot before volunteering for the Sonic Diver program. Her decision to transfer was prompted by a specific event during a mock battle: she was soundly defeated by a prototype Sonic Diver. Despite initially scoffing at the machine's skeletal appearance, this defeat impressed upon her the superior capabilities of the new technology, and she sought a rematch by joining the unit.

At the start of her assignment, Eika projects a hard, serious, and no-nonsense personality, a stark contrast to the other two pilots who were civilians. She holds discipline in high regard and is initially skeptical of her teammates' abilities due to their lack of experience. This strict exterior, however, conceals a more complex personality. She is known for being a tsundere character, often hiding her kinder feelings behind a brusque and moody demeanor. For instance, she has a strong aversion to being called by her first name, a quirk born from this personality trait. Despite her tough facade, she has a notable and somewhat humorous fear of ghosts, which causes her to act in a surprisingly girlish and frightened manner.

Eika’s primary motivation is to be an excellent pilot, a dream she has held since childhood. Her desire to follow in the footsteps of her father, a high-ranking naval admiral and former fighter pilot, is a central driving force. However, their relationship is severely strained because her father opposed her goal, believing that flying a fighter is too dangerous for a woman. This led to a rift, with Eika essentially leaving home to pursue her career on her own terms, refusing to leverage her father's status for special treatment. While she strictly adheres to orders and protocols, she is not afraid to question command decisions when necessary.

Her role in the story is that of a leader and a stabilizing force for the Sky Girls unit. She is the most mature and has the highest overall ability, possessing quick decision-making skills, fast reflexes, and superior stamina. She is the pilot of the H2N3 Raijin, a Sonic Diver that is notable for being the only one in the unit to use live ammunition as its main armament, giving it the strongest firepower. Her shooting skills are unrivaled among her peers.

Eika’s key relationships evolve significantly throughout the narrative. She begins by treating her teammates, Otoha Sakurano and Karen Sonomiya, with hostility and skepticism. However, after an incident where Otoha saves her when her synchro rate falls, she begins to open her mind and allows them to call her by name. This marks the start of her forming her first genuine friendships and learning from the very civilians she initially doubted. She also has a challenging relationship with another member, Elise von Dietrich, whom she seeks to discipline and guide. Eika takes care of Elise in various survival situations, and a pivotal moment in their reconciliation occurs when Eika selflessly saves Elise at great personal risk, expressing her sincere belief in their teamwork. Off-duty, she is known to rigorously maintain her fitness, often doing high repetitions on a pec deck machine, and has a revealed preference for spending her salary on expensive, high-quality chocolate.