Wait, you're all serious here ? You all know that a thirs character was inevitable at some point. You couldn't make a SoL work with only two MC interacting with each other unless they're the only two on Earth. Don't start to spout non sense as NTR when you barely know what it mean, they're just friends so they're no NTR. Omi is just here to make understand how each other see the relation : Hino see the teasing as a burden Koguma go into to go along with her and Koguma see their relation as special because she thought Hino was only okay with her doing that.
Seriously guys, you're all upset for nothing. Omi was never intented to make a serious rival, she was intented to make the relation grow and will probably be just a side character after just for a gag or two. Stop being so butthurt.