Maybe it's just me, but I'm getting the vibe of a "Sexy Onee-San" from the FMC (Oomiya-san). She's also a prime example of an Assertive Girl - she's set her sights of Nishima-kun and he ain't getting away until and unless she gets satisfied (if-you-know-what-I-mean).
And it seems that by the close, MC has achieved "Lucky Bastard" status (GRR)!
(ノ・_-)☆