Yotsuba Sugimoto

Main heroine of a PFP game. "Yokkyun" who seems to be drawn in a parody style of famed visual novel Kanon. She is the only recurring 'game' character of the series which is described by Keima as 'ideal heroine'. He seems to have huge affection and obsession towards her and whenever Yokkyun is insulted or ridiculed, he enters either extreme depression state or burst of anger.

(Source: Wikipedia: List of The World God Only Knows Character)