Rena wanted to be Ryuki's woman, but she didn't want that to happen immediately.
Rena knew that if she wanted to, then Ryuki would immediately accept her as his woman. Since Ryuki wanted to build a harem, he wouldn't say no to her, but Rena didn't want that.
If she simply goes and confesses to him and has sex like normal, then it would only be normal and nothing else. She wanted things to be different and unique, and something that Ryuki asked her; Ryuki got teased, and he wanted more of her.
Rena thought for a very long time. She wanted a reasonable excuse and something that could help her achieve her goal of doing something different that Ryuki would remember his entire life.
After many thoughts, she came up with an idea. However, this idea only works if Asuka agrees; otherwise, it fails.
After showering and changing her clothe, Rena went to Asuka's house.
She knocked on the door, and Asuka opened it; then they two headed to Asuka's room.