I have a fantasy story in which the lead character is designed as your typical bland everyman protagonist whose purpose is to give the reader someone to relate to and ask questions about the fantastical elements of the world to allow the reader to go through the journey of discovering the supernatural elements of the world along with the main character. However, while the character is initially meant to come off as a boring everyman for the reader to relate to, they are intended to gradually shed off their generic status with more unique traits as the reader feels more comfortable with the setting.
One of these aspects is intelligence. All of the other lead characters in the story come off as highly intelligent and scientifically minded, and it feels very awkward to me that the main character comes off as "the one dumb one" because they are shackled to the role of being the "generic everyman with straight B grades" to be relatable to the average reader. This is even worse considering that the character in question is Latino, which potentially sends off unfortunate implications that are the exact opposite of my intent.
Something I liked the idea of is that it's very strongly implied that the character is actually of above-average intelligence and while the character was an average student in grade school, this was more because they never felt the motivation to apply themselves and felt demoralized and overshadowed by their highly intelligent older sibling (who is Latina and high-achieving, and given the two are siblings it makes sense that they would both be smart). Thus, the character seems like an everyman but actually isn't.
My question is: how to I show to the reader that a character is highly intelligent when they aren't particularly academically inclined or "book-smart"? They also aren't super street smart in the way the term is also used, given they've lived a somewhat sheltered life in a safe neighborhood before being thrown into the plot, so that easy solution isn't really doable for me.