Even though it's still early on I simply adore Gate 7. It reminds me of why I love CLAMP's works.
Tachibana and Sakura are easily my favourite characters so far. Of the two I say Tachibana is the one I prefer. He has a very Subaru look about him and I like his serious personality. Sakura I like because he has a unique design and I like his easy-going nature which I find works well with Tachibana.
I ship them already. One thing that CLAMP has always been good at is adult couples who have a secondary role in the story. For X it was Seishirou/Subaru, Gouhou Drug has Saiga/Kakei, Kobato has Okiura/Sayaka, Tsubasa had Kurogane/Fay. And now Gate 7 has Sakura/Tachibana. How early is too early for writing fanfics?
Hana I find is completely adorable. I know, I know, this is yet again CLAMP doing a clueless, cute girl as a main character, but with CLAMP they eventually show that the clueless, cute girl has a lot of inner strength, which I enjoy. For example I really loved the recent Kobato chapters where Kobato's talking to Okiura. I suspect that Hana will be no difference. She can already take care of herself in battles and I think as the story develops (especially with regards her relationship with Chikahito) we'll see her personality grow.
Chikahito I like as a main character. He's a happy dork and so far there's no angst in his past which is a refreshing change. I think CCS!Sakura was the last time they did a main character who had no angst. Oh sure something could be revealed later on but right now I'm enjoying that Chikahito is free of angst.