Mion Sonozaki is the current heir to the extremely powerful Sonozaki family.
Mion has long green hair that is pulled back into a ponytail with a lifted fringe that drops on both sides. Her eye colour is officially green, but is often portrayed as a teal for contrast. Her breast size is, according to Rena when she pulls out the bra for her 'kawaii friend', is cup size 'X'.
Her schoolwear is a red lined white t-shirt, red tie, yellow waistcoat, and long ankle-length pleated skirt.
She loves collecting games from overseas - her locker is evidence of this. Stacked full of foreign board games, 'punishment outfits' such as maid costumes, swimwear, cat ears/balls, panda bells, 'little sister outfits' etc. for when a team member loses a club activity.
The club is run by Mion, who is the leader of the club, and also the class representative. She is highly respected in the community due to her social status and friendly attitude.
Mion's family is so rich and powerful that according to Ooishi-san, head of Okinomiya police force, that they could 'get away with murder in the streets of Hinamizawa'.
Mion has a twin sister named Shion Sonozaki who lives in the surrounding area of Okinomiya. Later we find out that their grandmother, Onibaba (current head of the family), disowns her. Despite the two twins living seperately - Mion being a total tomboy (on the surface) and Shion being a completely girly-girl - they are surprisingly close. Even though they may not show their sisterly love on the outside, since they are in constant rivalry over Maebara Keiichi, they know when the other is feeling low, and offer each other support whenever they need it.
The Sonozaki family have intricate underground tunnel systems that connect under hinamizawa and some of tesurrounding area - and own a lot of land. Their driveway being miles long and house being inpenetrable (almost!) by the police. In these underground tunnels the sanozaki family have a torture chamber - that was secretly moved down there once public practicions of the watanagashi festival became frowned upon. It is there as they believe that disowning he tradition will displease the towns deity - Oyashiro-sama, and therefore the torture materials and weaponary are still very much used down there.
Mion famously carries around a small gun in a holder under her left arm. She rarely uses it, and it is her weapon of choice. However, Mion's is of a much more casual appearance - being a country girl, it is common to see such items being carried.
Mion's secret is that she wants to be loved, and specifically, loved by Keiichi. She admires his ability to learn, and his academic stature. He is a generally all round-nice guy, and Mion, being used to only having female friends, admires that in the city grown boy.