ラブ・チェイン・ラブ・ジーン

Manami, a university student, approaches Kouichirou, a science graduate student as well as his older sister's arranged marriage candidate, to gauge his character for her shy sister. But, his impression is absolutely terrible! However, on a rainy day, they unexpectedly meet again...! (Source: Kobunsha, translated) Note: Includes one extra chapter.