Why is Kitamura called "Maruo" by Kihara?
Well, it isn't just Kihara who calls him "Maruo" - many of the girls at school call him that. In the light novel, it's pointed out early in volume 1 that various girls call him "Maruo" to be cutesy. In the anime, we see someone (I can't tell who) calling him "Maruo" in episode 14 near 4:40; and later, around 15:15, NANAKO Kashii refers to him as "Maruo" during a conversation with Kawashima and Kihara

Anyway: the reason he's nicknamed "Maruo" is that he bears a striking(?) resemblance (as Kihara notes in episode 2, around 5:00) to the character MARUO Sueo from the 90s-era show Chibi Maruko-chan:




The resemblance apparently extends beyond the visual - I haven't read/watched Chibi Maruko-chan, but Japanese Wikipedia states that Maruo, like Kitamura, is one of those do-gooder class-president types. 

There's no explicit mention of this in the light novel (unsurprising, given how weird Japanese media is about referencing other media - whence cometh MgRonald's), but some scouring of the Japanese internet suggests that the reference is fairly obvious to those who grew up while Chibi Maruko-chan was a popular thing.