If you're looking for manga similar to Keitai Shoujo: Triangle Spirits (Light Novel), you might like these titles.
Takeru and Sumika have been neighbors and friends since they were little, but when a mysterious girl arrives, and is found in bed with Takeru, their daily lives will be thrown into chaos.
For Aizawa Yuuichi, returning to the town where he spent his childhood has been an unusual affair. Yuuichi, it seems, has forgotten his memories of the past, including encounters with several young ladies who would love nothing more than to see him again. As the days pass, flickers of the past began to surface, revealing more and more of the dark events that once came to be. As the reuniting continues, Yuuichi soon starts to realize that his past may be far more complex than he once believed it to be...