I'm honestly not sure if the information I have to share will be enough, but like the title says I've been searching for years and it's driving me insane, so I thought I'd give Reddit a try.
(Please take all the information with a grain of salt, my memory is not the best...)
The series came out before 2015 (That was the last year I read it)
The books were read from right to left.
The books were in English, but I'm not sure if it was translated or originally in English.
It was a Comic/Manga/Manhwa/whatever (It had panels with pictures and speech bubbles)
I can't remember if the panels were colored or not, but I don't think they were.
The plot was something about a group of friends.
I think it starts off with them hanging around a skate park.
I think I remember them jumping over a fence? or maybe entering a public bathroom?
but some way or another they eventually end up in a fantasy world.
There were these animal spirit guardian warrior things that have basically chosen one of each of the friends as their champion/vessel.
From memory... The first book was about a Lion or Panther? and the cover was either blue or purple.
The first book was basically about the first friend awakening their power and training it.
I think they were preparing for a war or something? Against the dark/evil forces probably.
I think the 3rd book was green? and about a mermaid or something?
and in either the 2nd or 3rd book it seemed like one of the friends were going to be swayed to betray the group.
Unfortunately that's all I can remember right now...
I'm really hoping it's enough...
Thank you for your help in advance!
And if you need me to clarify or try remember something specifically more, just ask!