Risa's granfather appears in Chapter 50 - Chapter 54 of the Love★Com manga. He has spent most of his adult life traveling abroad and gathering pictures of beautiful men during his travels as gifts for Risa. After first meeting Ōtani, he hires call girls in an attempt to get Ōtani to cheat on Risa and ultimately break them up. It comes out later that the reason he doesn't like Ōtani is due to his own height complex with his late wife. He was shorter than his wife and she became ashamed of their height difference, which is something he didn't want for his beloved granddaughter. After talking with Ōtani about this and getting reassurance that he and Risa are not ashamed of each other's height difference, he gives his blessings to their relationship and backs off.
Risa's grandmother is only briefly shown during flashbacks on pages 116 - 118 of Chapter 54 of the Love★Com manga, and Risa looks exactly like her. It is shown that her grandmother was very tall, standing taller than Risa at 6'5" (195.5cm), and constantly hitting her head on things. Her husband fell in love with her because of her confidence, which he thought made her more beautiful than anyone else, a "5-plum." But after marrying, she became bothered by being taller than her husband and would walk apart from him in public or crouch down to appear shorter. According to Risa's grandfather, she was always sickly and died quite young. It's unknown if Risa or Takato ever met her.
Like most Japanese mothers, Risa's mom is a very polite women. She's very tolerant of her daughters shenanigans and is only seen raising her voice when Risa mouths off or does something stupid (like putting her grandfather in a headlock). She often worries about her daughters obsession with video games, but is the first to step up to please her with things like an all-Japanese breakfast every morning.
Risa's father doesn't make many appearances, but he is one of the first to hound Risa for her height after they find out Ōtani is shorter than her.
Takato Koizumi is Risa's younger brother. He resents his older sisters antics, often speaking down to her and cannot understand how he could be related to "such an idiot." On the other hand, Takato looks up to her boyfriend, Ōtani, and tries to convince Ōtani that he is too good for his sister. He becomes increasingly more aggressive towards Risa and her friends after he fails entrance exams to his school of choice and has to attend Maido Academy instead, a school he thinks is for idiots like his sister. In Love★Com Two, Takato lightens up a little and starts to show a goofier side which is very much like his sister.