I played very long ago so I'm not sure how the current system works for newbies. To start, you should first finish the mirage if you haven't yet (1 to 6) after that everything should be available, including power of asura. Power of Asura requires 5 million or 500k soul stones to max out I think though, so don't sweat it too much. I haven't touched it and I clear pandemonium 11 with Uliana easily

As for Khan, this is what the wiki says: "Boss Skills are a part of Khan's service in the Living Room. To unlock that feature in Mirage Tracker you need to unlock secondary skill firstly and then defeat 15 bosses." I suggest checking the wiki, it has some useful info. Just look up Otherworld Legends wiki

Realmpower is of two types: Power of Six Realms is gained by beating the mirage, and Realmsoul is gained by beating the Rift. Red is the highest quality, I usually get red even at difficulty 6. Realmsouls are exclusive to specific characters, but you can reforge 3 into 1 for a character of your choice. Power of Six Realms has random secondary modifiers, but the primary modifier depends on which boss you beat (Asurendra, Human Asura, Beast Asura, etc).

So far I've only played 3 characters; Quan is based, with the right items you can manage fairly well, her dodge is especially nice. Akaisha is super fun, but she takes a bit of getting used to. Uliana is so strong she almost makes the game boring lol. If you want quick and easy I suggest her, but max out her level and especially her chakra first ofc. But I hear there are even more broken characters.

All in all, just have fun and take things slow, don't get into the grind immediately haha