@Faryshta
Thats still tell since we don't know how much is he reading it, he just turns some pages and thats all we actually see. We don't know if he is passionate about it, if he actually reads it or if he is just and edgy teen.
When you see someone reading a book, a normal thing to assume is that he reads it because he likes it. If he was reading it just for show, it would have been established early on, and even then his choice of a book would have been quite telling.
That's quite a major point that he has no friends with whom he could talk about it. Another major point, reinforced in this chapter, is that this hobby is not exactly a popular discussion topic.
having any active participation on his hobby like for example assisting to an expo, learning some skill
Does anyone who, say, likes Tolkien books, have to attend conventions or roleplay as Gandalf? Must they learn swordplay or magic to prove the world that they truly like Tolkien books?
And it's not like we see that his hobby influences him enough to make him misuse a paper knife twice or misinterprete his feelings towards Yamada as "murder intent lol".
or simply fucking mentioning it
And into the bin goes your "show don't tell" rant.
Lets put Yamada into perspective. Instead of having the first page saying "Yamada is an actress" we see her actually appearing on TV, actually practicing skills related to her passion, we see her being depressed about not being able to assist to a photography session, we seee her acting a famous trope scene of "your loved one turned into zombie".
If it was a manga about a struggling writer and a novice actress, it would have made more sense to dwelve deeper into what Ichi does when Yamada is not around. As it stands, this is a manga about growing relationship between two teenage dorks, so whatever are Ichi's hobbies, they are at most secondary to the primary plot.
And since the very first message I specified I am talking about his individual personality aka what makes him unique.
Oh, then there's no point in arguing about your personal definitions of things. I will say though that I would rather have BokuYaba be about humanlike characters rather than about characters made deliberately quirky so that they qualify as "unique"