Anri's canon, specifically, is from the manga Durarara!!, or DRRR!! for short. If you've heard of this series or you're interested in reading it and don't mind spoilers for this character, then please do read on!
Character History
Pre-Mika Harima/Dollars Arc
When Anri Sonohara was a little girl, estimating very young to about ten years old, she experienced something most little girls do not; physical abuse.
Her father basically acted like bastard when the family business was failing--this antique shop he co-founded with his spouse. He projected his disappointments and frustrations on both Anri and her mother.
Overtime, the abuse continued and had gotten so bad to the point in which one day Anri was pinned to the ground with her father's hands clenched tightly around her throat. Through her glasses' frames, she saw a figure. The figure moved swiftly, and killed her father. When Anri was able to see her father's murderer, the woman smiled down at her and held a sword blade to her throat.
"I love you, darling."
That was all her mother left her before she commited suicide after killing her husband. Anri, covered in blood, stood and approached the weapon that had taken her family's lives. As she gripped it, she became one with it.
This was the demonic katana, Saika.
At the moment both parents fell bloodied and dead in front of Anri, she quickly recalled that there were news reports about sightings of somebody on the loose with a katana attacking anyone that came within their range. She realized her mother had been the "serial slasher" the entire time when the Sonohara antique store was going out of business.
Saika's power was "to cut"; "to love". "To love" all of humanity. The blade posessed whomever it could to carry out that purpose. However, the incident had robbed Anri of her ability to love. With this, Saika's power was negated. Anri used this to her advantage: she began to control the blade.
Mika Harima/Dollars Arc
It was years five later that this ability became useful. At first, it was a quiet life. Anri's inability to love, as she puts it, made her a 'parasite'. She was, for most of middle school, Mika Harima's foil. Or at least, that's what she believed. Anri believed that Mika used her to look good, but Anri claimed to not mind. Because, after all, she was a parasite. Mika was just her current host. And yet, she cannot quite define the relationship between herself and Mika. She cared enough to look for her, at least, but still. Was what she felt for Mika Harima love? Was it just concern for her host? Love is a strange emotion, and one Anri rarely, if ever, dealt with.
She attended Raira Academy, without Mika, who had gone missing after sending everyone she knew a text. Anri met Mikado Ryugamine and his friend Masaomi Kida. Unknown to Anri, her new friend Mikado was leader of a gang called "Dollars". The "Dollars" were--are-- part of the color gang faction. Unlike the Yellow Scarves and Blue Square, however, "Dollars" had no color. The simply existed. They were, in a word, "invisible" due to their lack of color.
Saika Arc
It was shortly after meeting those two boys that The Slashings began. (NOTE: to know more about Saika herself along with her powers, read through the Saika Glossary.)
The Slasher was an unknown person who simply cut people. The victim lived, though they suffered physical damage. Anri was attacked by the person responsible for the slashings.
This person came for her, eventually-- she was being bullied and the bullies were attacked. She witnessed the crime and was taken in for questioning, but nothing more. Nothing less for that matter, too. Anri wasn't necessarily unphased; she had, after all, witnessed an attack, but she herself was unharmed.
A man who was a third rate tabloid reporter who had been slashed. Later, he was sent to kill Anri, or pursue the most strongest man in all of Tokyo with a short fused temper; Shizuo Heiwajima. Both Shizuo and a dullahan without her head, Celty Sturluson, came to Anri's
When this tactic failed, the Mother of the possessed tabloid writer came for her next.
Anri opened her door one night to meet Haruna Niekawa.
Haruna had been a student at Raira Academy the year before, and had entered into a relationship with a teacher--supposedly--a teacher whom had been sexually harrassing Anri. Haruna explained to Anri that she couldn't forgive her for taking "her" Takashi Nasujima away and revealed herself as The Slasher. Before she attacked Anri, the doorbell rang.
An informant named Izaya Orihara had sent Nasujima to Anri's doorstep at that exact moment. While Haruna went after him, who ran away upon seeing her, she sent five of her "Children" on Anri. Anri defeated them, and went after Haruna next.
Anri revealed herself as the true Mother--the owner of Saika--as the katana blade slid out of her arm.
Anri defeated Haruna, taking control of Haruna's Children. Once this was all over, she put Saika back into herself and went to the hospital as the final Slasher victim. For a little while, she stayed there and recovered from the cuts and slash marks from Haruna's knife attacks...
Personality Traits
Imagine that, in the back of your head, there are constant haunted whispers, echoing, "Cut. Love. Cut, cut, cut. Mother. IloveyouIloveyouIloveyouIloveyouIloveyou..." Wouldn't you go insane from that?
Welcome to the everyday life of Anri Sonohara.
She is a girl who deals with the fact she has immense, possibly destructive power; but you would never guess it looking at her. She's shy, quiet, and introverted. You'd miss her if you weren't looking for her, not to mention she's just under five feet tall.
She is a mess of contradictions at best. It's simple to see that, at her core, she's a simple teenage girl with a lot of problems. The shock of her parents' death has had a large impact on her. Certainly, the abuse dealt to her by her father as well. The past has weighed heavy on her, and given her a unique outlook on herself: Anri calls herself a parasite.
This originates from a group of girls in middle school that would gang up on her and scorn her for "leeching" off of Mika Harima whenever the two of them together. After hearing this over and over on more than one occasion, she knew it to be very true. Those girls were right, but Anri had already chose to live this way long before they began to prey on her and treat her like dirt. Anri took their harsh words and getting shoved or knocked down without fighting back.
It's strange though; Anri was also okay with Mika having her close to look good, because it was a fair trade.
Anri has a strong sense of what is fair, and goes out of her way to make things even on both sides. When her teacher Nasujima was approached by The Black Biker (who she later met and came to relate to as Celty), she used Saika to attack The Black Biker and fend her off. All for the simple fact that Nasujima in question had helped her get into the same class as Mika.
You wouldn't think it, but Anri is a surprisingly brave young lady. She's a bit bold as well. She has no reason to apologize or make an excuse, so she simply doesn't. Her lifestyle would be called pitiable or disgusting by most, but Anri does not, and will not, change it. Anri chose to live this way to use as a medium to hopefully find her place in the world.
She has no problem manipulating a girl who was capable of controlling Saika, and letting her cut her a bit in exchange for defeating the girl herself. To some extent, you could call her frightening. When she has her mind set and cleared, she has no problem doing what she needs to. Anri could, maybe, be described as "hands off over protective". You might not know it, but she's risking her life for yours.
Anri has an interesting view on love. She says she isn't capable of feeling love-- yet this is untrue. She has blatant disregard for herself. For example, she is almost attacked by Saika, but doesn't move an inch. She understands what's happening, but defending herself isn't in her mind. But somehow, she is capable to whipping a sword out, and determinedly going into a fight all for anyone she'd care about. It might not be a romantic kind of love, but it's defintely a show of love when she can come out of her shell like that for you.
Yet, she is a girl who will look away and blush if you compliment her. She didn't react when Nasujima hit on her. She doesn't react to being picked on, and she's also socially awkward. When she feels threatened, she mentally removes herself from a situation, like when the bullies were picking on her, and when she was face to face with Haruna. Anri blocks everything out: vision, sound, probably smell too. She completely removes herself from the situation, repeating that she isn't involved, and this will resolve itself on its own. She visualizes herself staring emotionlessly at everything that's going on as if it were all a framed picture or painting. It's a terrible coping method, but it's also proof that Anri protects what she loves.
When that fierce fighting spirit isn't triggered, Anri just a nervous, socially awkward girl. Despite her shyness, she is capable of carrying a conversation. Her voice sounds like a whisper, but you'll hear it, and it might even be surprisingly friendly and polite.
Self consciously, Anri completely rejects the idea of love and hides behind Saika. Anri finds love, in a way, terrifying. Saika is Saika and she loves humanity, but look at Saika. She's a crazed demon sword. Look at Haruna, and look at Anri's past. Love seems to be more bothersome than any other emotion, so she rejects it for that simple fact.
Yet Anri craves a happy family life, something she never quite had. She used to have dreams about being with her family; smiling, laughing, and overall having a good time. When the slashings first caused by Haruna had stopped, the dreams at the same time had gone away. Although, she still craves for such happiness.