Akiho Sairenji, Haruna’s older sister, shares an apartment with her after their parents’ relocation due to their father’s job transfer. At 19, she balances college studies with financially sustaining their household, aided by an unspecified job under an "editor in chief" superior. Tall, curvaceous, and striking with long purple hair and matching eyes, she mirrors Haruna’s features but projects a more mature, developed presence.

Her personality blends responsible maturity with mischievous sibling teasing. She approaches dating with nonchalance, engaging in transient relationships like her half-hearted involvement with Kotegawa Yū, often rebuffing his invitations with flimsy excuses. Despite attracting numerous confessions, she consistently declines, reflecting her current disinterest in romance—a trait Haruna recognizes. Akiho keenly intuits her sister’s emotions, such as detecting Haruna’s unspoken plans to meet Rito Yuuki. While she actively fosters Haruna’s affection for Rito through playful prodding, her teasing sometimes oversteps, sparking mild tension due to Haruna’s aversion to public embarrassment. Their bond remains strong, balancing encouragement with lighthearted provocation.

The Sairenji family’s names follow seasonal themes: Akiho’s "autumn" (秋) kanji complements Haruna (spring), mother Natsumi (summer), and father Fuyuhiko (winter). Residing with Haruna and their dog Maron, Akiho’s work commitments leave most pet care to her sister.

Featured in main series installments and the sequel *To Love-Ru Darkness*, Akiho maintains a consistent role as a supportive sibling figure, her playful maturity and casual romantic disinterest remaining unchanged throughout narrative arcs.