I returned the gesture tepidly. I was pretty mad at him then, and I stayed that way for a couple of weeks. The language and manner of the narration is, of course, chosen according to the character. Ranma uses somewhat rough speech, whereas someone like Kuno would use much more formal language. Don't go overboard with weird language, though.
First person is an example of a limited perspective.
The narrator character can only tell us about things that he knows about. Akane being uncomfortable is his inference, and not a fact, though given the events of the story it's very likely correct. Note, though, that the typical first-person story is being narrated some time after it happens. The main reason is that you need a narrator character who can be present for all the scenes you need to show. It works well for fics where the spotlight is mainly on a single character, not well at all for stories that jump around to different locations and feature a large cast, like many epics.