The Stig is a real life racing driver who participates in numerous car tests and silly challenges in the British Auto Magazine TV show Top Gear.
He remains silent, never speaking and occasionally engages in the goofy antics of the three hosts.
Part of his lore is that he is seemingly invincible and capable of things no other human can achieve, three real drivers have played the Stig, Perry McCarthy, Ben Collins and Michael Schumacher, no one knows who plays him currently but it's not unusual to have him being played by multiple drivers depending on location and availability and in the case of Schumacher, the owner of the car they were testing.
So when we get to Bakuon, we meet Raimu Kawasaki.

Raimu is the silent protagonist of the school motorcycle club, she communicates through body language and written signs never taking off her helmet, she has twin braids, a bow, a Simpson brand helmet and a sailor fuku school uniform, however it's revealed quite early on that she went to school with the teachers, even if you consider the teachers young, Raimu should be close to 30 years old, yet she doesn't age at all, making her an anime Stig, it's a fun little tribute to a character with origins in the UK, whether that's intentional, depends who you ask but I have no doubt where Raimu Kawasaki comes from.