this " I " looking this, is it a character or grammatical symbol? https://imgur.com/gallery/mRZZ6PG thank you
It's the manga way of extending letters in a word. Say "Oh" : O-h = Oooooh What it is... I'm not sure what it's called.
It's called the Choonpu. And to correct jawz, this is not a "manga way of extending letters" but a common character in katakana. For example, ramen is written ラーメン
That is an interesting question. Japanese has a traditional vertical format called tategaki. I wouldn't be surprised if it was originally vertical, but later rotated for horizontal Japanese to stand out more or something. There's a decent chance a historian with a focus on premodern Japan would know a definite answer, if this interests you.
I always thought it has to do with how they used to write with a brush. As you write you never want to change directions or move the brush too much + the characters are usually written in a way, that you end up somewhere below or below and to the right of where you start -> it is natural to just move the brush vertically next if you are writing vertically or from left to right if writing horizontally (especially so in a cursive script). Not that it stops people from making signs with choonpu written wrongly (and it looks silly, at least to me)