Here's your sister's description.

Old old name: Anna Ikax

Old name: S/n Holat-Raizo

New name: S/n L/n

Hair color: Blonde

Hair length: Long

Eye color: brown

Nickname: Little sis, Munchkin, Sweetheart (All by Y/n), Short stuff, Blondie (By Classes A-D and Karma), Y/n's sister (by Ritsu) S/n/n (By class 3-E) Shortstack (by Karma)

Age: 15 (at the start of the anime)

Birthday: Christmas

Roll call number: E-30

Preferred weapon: Anti-Sensei Knife

Personality: Calm, childish, happy, a little clingy, nervous

Appearance: Female school uniform (She says it's too snug around her chest 😏), Green gloves, White eyepatch over left eye (gone by Baseball time. Same situation as Y/n's eyepatch.), brown right eye, long blonde hair.

Species: Human?

Likes: Her brother, Ritsu, her friends, exercise, flirting with Y/n (She isn't blood related so it's fine.)

Dislikes: Jelavić (She was ready to kill her when she tried seducing Y/n), Korosensei (She calls him "Perverted Rated-R creature". She also doesn't know what Y/n knows so she actually tries to assassinate him.), Terasaka, Yoshido, and Muramatsu, her friends and brother getting hurt, seeing Y/n sad, perverts being perverts, people trying to flirt with her, people trying to flirt with Y/n.

Friends: All of class 3-E (except for Karma, Terasaka, Yoshido, and Muramatsu), Karasuma, and Y/n.

People she's closest to: Y/n (She loves her brother very much.), Nagisa (He's the main character, so obviously she's his friend.), Ritsu (S/n understands how Y/n feels about her. She's willing to share him as long as Y/n treats both girls equally.)

Abilities:

Superspeed - If S/n attaches a string to an Anti-Sensei Knife, she can swing it so fast that she can fly as long as her arm doesn't get tired.

Actual superspeed - She inspired Y/n to exercise. Like Y/n, she continually trains her legs "off-camera" to be able to go as fast as Korosensei, maybe even faster. At the start of season 1, S/n secretly can clock at Mach 5. However both siblings can go faster.

??? - a secret ability for her that I'll reveal once I reveal it in one of the chapters.

Endurance - You can regenerate your energy at a superfast rate.

Fever: The polar opposite to Y/n's "Chills" ability. Instead of keeping her body cool, S/n stays warm, preventing her from getting any cold related sicknesses or injuries like frostbite. Anyone who stands near S/n will feel warmer than normal. If Y/n and S/n are near each other, Fever and Chills cancel each other out.

Heatwave: Like with Y/n's "Freeze" ability, if she gets angry, the temperature of any room she's in will skyrocket to 100° F until she calms down. Unlike Fever and Chills, Freeze and Heatwave DO NOT cancel each other out, meaning that if both L/n's are angry and you're in the same room as them, prepare for a super high chance of hypothermia and heatstroke at the same time.

Backstory: S/n was an accidental birth, as by the time she was 3, her parents put her in an orphanage, where she stayed until she was adopted at 8 years old by M/n Raizo and D/n Holat. While at her new home, she met Y/n Holat-Raizo and she felt a connection with him. Unfortunately, S/n saw how Y/n was abused by M/n and neglected by D/n. She was forced to keep this to herself as she was threatened with the orphanage if she told anyone. Despite this, S/n still spent time with Y/n and cleaned his wounds. One day she heard that Y/n got expelled and sent to 3-E so she decided to spread lies about the other students, which she made sure were  very see-through, causing her to get the same punishment as Y/n, the only difference being she got her left eye shot out. Before she went to bed, she overheard D/n and M/n planning that they were gonna make Y/n and S/n wish they were never born. S/n quickly packed her bags and woke Y/n up. She told him what their parents were planning to do and that she got herself expelled and sent to 3-E as well for her lies. She told him to pack his bags as she wanted Y/n to run away. Y/n told her that he wouldn't go without her, which she said she knew and revealed her already packed bags as she planned on going with him. During the night, the two siblings packed the few things that Y/n had and ran off to find somewhere else to live. M/n and D/n didn't even care that they were gone though, and just continued their lives as if they had no kids. Eventually, Y/n and S/n found a house and after a while, they made it their new home. They even changed their last names to L/n.