If you ran into Haruka, say, at the Crown Game Center, she'd most likely be playing the racing game. After winning (and beating the current record), she'd start a conversation--if you're a pretty girl. She'd probably act in a flirtacious manner. If Michiru's not around already, she will show up in a minute or two and be annoyed at Haruka for flirting (if you're female), like when Haruka first met Usagi and Minako. If you paid the least bit of attention to Michiru, however, Haruka would be a little, er, hostile. Especially if she doesn't find you cute. (Funny, you wouldn't expect someone so self-assured to be the insecure type.)
No matter what, though, you would most probably never get to know the real Haruka. Her cocky attitude is a front that you will never successfully penetrate to reveal the thoughtful, rather tortured individual beneath. Only her partner (Michiru--what, you thought it was Seiya?) completely knows her; she lets down her defenses for no one else, albeit Usagi to an extent, although she certainly drops her reserve around Hotaru and probably Setsuna, too during the time the three are raising Hotaru. Setsuna certainly has a fair understanding of Haruka because of her all-knowing nature, but we can't tell whether Haruka ever opens up to her the same way she does to Michiru. Certainly, no one but the senshi of the sea really knows her.
Not many details exist regarding Haruka's background before before her awakening as a senshi. One thing is clear: as she participated in fast-paced sports to "be the wind" and flee her destiny, she never let anyone get too close to her. Speculation runs rampant when considering what might have caused her to build barriers around herself as she has, but it all leads up to the same fact: before Michiru came along, she was completely on her own with no sort of emotional support. Given the circumstances, opening up to Michiru was a big risk; her insecurity regarding anything that threatens to destroy Michiru's interest in her stems from the fact that she has grown quite dependent on her partner. Nothing shows this dependence more clearly than her actions at the Marine Cathedral, as Shilpa addresses here. Although her dedication to her mission remained, her grief-stricken actions were quite illogical (what does Shilpa mean? Read that page, she's not going to repeat herself!).
Haruka is quite an emotional person, especially when compared to Michiru. Her emotions surface more easily and are usually more evident than those of her mysterious partner. If something bad happens, it's usually visible on her face. One example might be when Usagi's heart crystal is taken out (episode...er...get back to you on that one....). Uranus says that Odango Atama must be sacrificed for the good of the mission, but she's clearly worried.
Tenoh Haruka is an amazing character. Strong but vulnerable, dedicated but tortured by the necessity of her actions. To an outsider, she is a brilliant student and champion racer, cocky and flirtatious. If they could penetrate her front, they would see much more...