Unofficial biography, made by me. Contains lots of spoilers for the series. I will make a spoiler-free biography soon. Largely incomplete.

Name: Ayumi Miura (三浦 歩美)
Nickname: Matsu Miura
Age: 17
Birthday: September 14th
Eye Color: Carmine
Location: Kyoto (moved from Chiba)
Hair color: Blue
Favorite color: Violet
Personality: Timid and shy
Pronoun: Watashi
Voice Actor: Nozomi Higetsu (緋月 希)
Debut Episode: Season 1, Episode 1
Nicknames: "Imouto-san" (Azusa), "Matsu-chan" (Takane), "Matsutsu" (Akane)
Current status: Alive
Name Meaning: Her first name means pine (松). Her last name means three (三) and inlet (浦).
Quote: "Hello there... I am Matsu Miura. Azusa is my big sister... do you want to be friends...?"

Matsu Miura is a 17 year old girl, who is the timid little sister of Azusa. She wants to grow up to be an artist like her big sister. She loves anime-style art pieces, and often stays up browsing anime galleries at night. She is a really skilled artist, and often takes anime art commissions online. She is also a little bit of an otaku, having a body pillow of her favorite anime waifu and often going to anime stores to get merchandise. Azusa doesn't mind it, but she minds it when Matsu is being inappropriate about it.

She is a big nature lover as well; she has a garden in their backyard, filled with many types of plants.
When Azusa introduced her friend, Takane Shijou to Matsu, she hid; she was scared of Takane, even calling her an alien. Azusa eventually convinces her that Takane wasn't an alien, but she was still scared of Takane. Takane introduces her to her little sister, Akane, but Akane tackles Matsu and goes crazy. Takane tells her to calm down, but Akane wouldn't even listen to her big sister this time, making Azusa scared of Akane, and Matsu extremely scared of girls from the Shijou family.

Her best friend is a loner named Chihaya Kisaragi, a teenage schoolgirl who would prefer online friends instead of real friends. Chihaya was kind of an otaku, and often they would share their interests. They went to the same school. When Chihaya became a scientist, Matsu congratulated her and planned to hold a congratulations party, until one of Chihaya's experiments went wrong, and Chihaya died a few days later, leaving Matsu devastated. She went to her funeral, crying her eyes out, along with Takane.

When Azusa got married to her sweetheart and had a child, named Shizuka, Matsu was seen to be very happy. Matsu tries to find the opportunity to play with Shizuka whenever she can.

Facts

Her favorite food is savory Japanese food, and some Korean ramen as well.
She loves playing Pokemon, and her favorite pokemon is Lapras. Her favorite Pokemon type is ice.
She might be either lesbian or bisexual, as she has shown to own at least 3 female body pillows, and her being a fan of more anime females than males. She also admitted to her big sister she had a crush on Chihaya, but was too afraid to tell her.
Her original last name was Hamasaki (浜咲, which means beach blossom). However, probably due to there being an Ayumi Hamasaki in existence (with different kanji, however), their last name was changed to Miura... although there is an Ayumi Miura; you probably shouldn't google that...
Her original design was shown to have much shorter hair and paler skin.
She is exactly as old as Ayumi and Ayumu Tsukihama; they were born on the same birthday and year. They were created by me, by the way.
She strongly disagrees with Akane's plan of being an "IDOLmaster," as she said that Akane wouldn't succeed in real life due to her being too "freaky."