X Equals: X wa Nazotoki ga Osuki

Kai Kunieda is known as "Holmes-sama" due to his superior power of deduction and slightly quirky personality. Kii Hosaka, a girl who secretly has feelings for Kai, becomes his "partner," but it ends up that only means his partner in detective work... Nevertheless, Kii tries her best to express her feelings for him as they go around solving the mysterious incidents that happen around them.

(Source: Shogakukan, translated)