By the way, I don't know about you guys, but I'm calling this a harem since there is more than one girl gunning for our main character.
Well, I won't go so far as to call this a harem. So far, only Honoka (Excluding the imouto) has shown feelings for Tatsuya in a romantic way. Though Mayumi and Lina is also interested in him.
Sadly, Mayumi most probably thinks of Tatsuya as a little brother. Honestly, I don't actually mind there relationship going that way (despite being a Mayumi fan). I think Tats need someone to confide in or maybe to take advice from(I know he's a genius and all that but Mayumi has more experience on life than him, not to mention she can be an inside connection to him and she's very mature and understanding than Miyuki). With her easygoing and understanding personality she can be a very good support pillar for Tats.
As for Miyuki and Erica knowing most of Tats' secrets. Knowing secrets about someone else doesn't necessarily point a relationship into a romantic path. They can also be very good friends. Anyone can know some secrets about someone, but not everyone can understand them or empathize with them. Which Lina should be able to do to a good extent and Erica to a certain degree. I'm excluding Miyuki because I think she can hardly understand who Tats is due to her upbringing. She might support him with her soul and agree with him on everything, but i doubt she'll be able to empathize with him or understand him, for that matter. As she is now, she will never even question Tatsuya's intentions or give any suggestions or advice to him. He will have to do everything by himself. That's a very big burden on someone. No offence Miyuki fans, but to me, she's more of an accessory that'll always agree with Tats on every damn thing he does.
As for Tatsuya x Erica, there are hardly any flags raised in that direction. C'mon, just think about all the screen time Erica and Leo had together. Those two will make a great couple. Despite their bantering they actually have very compatible personalities. Actually, I thought the author's making them in to a couple since the vol 1. Erica admires Tatsuya, but I don't think she has any romantic feeling for him. And all of his friends wants to be a support for Tats, not just Erica.
Anyway, since Mizuki and Mikihiko have been coupled, that leaves Leo with Erica and Shizuku. Anyone knows any relationship between Leo and Shizuku is none existant. That leaves him with Erica, who he already has a good relationship with. At least, it's very clear that the author is pushing them into a some kind of special relationship (which is most probably a romantic one).
In the end, I can only hope the ending won't be like Oreimo. *fingers crossed*