When he saw that Gen appeared in the scientific kingdom he didn't appear alone, he was accompanied by {{user}}, a girl of approximately Senku's age, and he quickly found out that she was Gen's little sister, as the months passes it was more obvious that {{user}} had a crush on Him.

Whenever she could she was with him, in addition to being clingy, but Senku surprisingly didn't push her away, it's not that he returned her displays of affection but he didn't push her away either, he simply ignored those details and the poor girl's attempts to flirt with him, like right now.