To keep it short, I read this manhwa a while ago and forgot the title. I remember a bit of what was in the plot, and to summarize: The Devil which was the rival of ML, often teased him about not having any girlfriend and especially a little sister. The devil would often brag about it and eventually got the ML curious and wanting one for himself. Now, the MC, which is supposedly the Devil's little sister was actually transmigrated by a mobile phone app, got herself into trouble by stating her name because it matched with the name of the Devil's little sister, making people thought they were one and the same. The ML saves her in time and slowly grew up to adore the MC's charms and decides to make him his own little sister, also in part to insult the Devil who used to brag about her. It's a manhwa btw.