|You do NOT want to piss her off. Just a thought.|
Heavy spoilers beyond this point. You have been warned.
How do I begin to describe Mikoto? That's what we're here to find out, isn't it? On the surface, she seems like a perfectly normal girl who happens to be one of the strongest espers in Academy City. So what else could there possibly be to this girl? There's quite a lot, actually. Let's look at how she interacts with others first.
Well, the interaction we see the most with Mikoto is with her roommate and presumably her best friend, Kuroko Shirai. Kuroko is certainly... friendly, to say the least, going to great lengths to protect and assist her, even in the toughest of situations, as we see during the Daihaisei arc, when she comes to assist Mikoto during a hostage situation, despite having her memories tampered with by Misaki Shokuhou. Kuroko would easily put her life on the line to protect her Onee-sama, and Mikoto would do the same for Kuroko. To Mikoto, Kuroko is a friend and confidant, especially after the events of the Sisters story arc, where Mikoto would stubbornly refuse outside help.
Another frequent interaction seen involving Mikoto is those with Ruiko Saten, a Level 0 friend of Kazari Uiharu. Like Kuroko, Ruiko is a source of comfort for Mikoto and one of her friends, willing to sacrifice herself despite her lack of an esper ability. Ruiko learned about working hard from Mikoto during the Level Upper story arc, and Mikoto was reminded of the hard work she had to put in to get to Level 5. Both girls seems to motivate each other throughout the story.
With such positive interactions, there are most certainly negative interactions, the most obvious of which is her brief interactions with the strongest esper of them all, Accelerator. Accelerator famously was put through a project in which he was set to fight 20,000 Level 2 clones of Mikoto. To this day, this disturbs Mikoto greatly, because it was her naivete at a young age that led to the creation of said clones. Her brief interaction with Accelerator during the Sisters arc was a combination of fear and anger, and this interaction lead to her finally accepting Touma Kamijou's assistance.
Mikoto's interactions with the clones otherwise known as the Sisters is interesting, because her feelings toward them began as regret and hatred, but evolved into the same sort of defensive care she would engage in for Kuroko or Ruiko. She's shown to be particularly defensive of #10032, otherwise known as Misaka Imouto. It's yet to be made particularly clear why #10032 gets such love from Mikoto, but perhaps it could be because 10032 was the first clone to survive her experiment.
Interactions alone do not make a character, so let's dig deeper into Mikoto herself.
Mikoto is a Level 5 electromaster, and is not afraid to make it clear to you. She'll take out the city's power grid if it means frying your ass for pissing her off. Another one bites the dust, as they say. Her favorite intentional target of her power seems to be her rival, Touma, whom she has yet to beat as of manga volume 9. Another frequent target is her friendly roommate Kuroko, often punishing her for trying to be close, to keep it PG. However, the latter usually incurs the wrath of the Dorm Inspector, who straight-up instills fear in even the toughest people.
Now that we've looked at the girl herself, let's summarize everything I've discussed.
Overall, Mikoto is the badass oujo-sama archetype, but with very few of the tropes that accompany the oujo archetype. She's shown to like extremely girly and childish clothing and hobbies, something Kuroko chastises her for multiple times. She's also shown to be very brash, which is, y'know, not exactly lady-like. And then there's the shorts she wears under her skirt. She claims it's so her panties don't show, but Kuroko ain't buying that. She's deeply motivated to fight once her friends are involved, which, since 2 of her best friends are in the Judgement task force, is rather often. All in all, Mikoto is usually just a girl who wants justice to be served and won't let anything stand in her way to make that happen, as she would presumably have made Kuroko her enemy in order to shut down the Sisters project. This makes her a very interesting character, and probably relateable to some, which is a key part of a well-written character.