I personally love character design, so if you want this to be a collaboration and feature different people's characters, I'm in. 
If you feel like you just don't have enough ideas but would actually prefer having your own characters, then maybe watching those videos and giving it a go yourself might be better. It really depends on your reasons for asking us to design your characters.
But, if you do just want to make it a cool collab, I agree that it would be helpful to add a bit more information - like what age group the characters should be, if they have to have a certain educational background, their current situation (are the characters all students at the same school or uni? Co-workers? Teenage runaways? Adults who don't all work in the same place so we're basically free to pick any background and workplace? Retired elders?).
Some more information about the plot and themes you're comfortable or uncomfortable with would also be helpful - for instance, a character with some pretty severe trauma might not be suitable for a quickly developing romance (of course everyone deals with trauma differently, but it's difficult to write about a traumatised character who jumps into a relationships, trusts easily, etc, without making it seem like they're unbothered by their trauma, or like the trauma is just there to facilitate the love story if it mostly comes up when the love interest can comfort the character - it's not impossible, but tricky and not everyone would be comfortable with that, and that's just one example).
What kind of world do the characters live in? What kind of society? I know you said modern-day, I'd assume modern-day Korea, but since your story features mythology, fantasy elements could change some things and make that world differ from the one we know. Characters are shaped by the society they grow up in, just like real people, so that would be useful information too.
And then, of course, how much do you want people to go into detail? Do you want us to think of everything, a backstory, detailed personality, relationships, etc? Or would you prefer some more vague ideas you can build on to make the characters your own? Would you like sketches as well? If so, does the art style matter? Whether it's coloured or black and white?
In short, a synopsis of your story some more info on what exactly you want/need would be helpful. But since I really love designing characters, I'd still like to help you if you end up still wanting that help. 
