First of all, what does it mean to "main" a champion? In simple terms it means you are very good at a champion. As you can see, that definition is really vague so I'll try to explain further.
There are 100+ champions in LoL and summoners tend to have favourites that they play a lot. They pick them based on certain criteria like skill set, abilities, mechanics, difficulty, etc. Now maining a champion isn't having a Champion Mastery level 5 icon. It's not having all the skins of that champion. It's not playing 300 games with a champion and winning only 133. No people, that's not "maining" a champion.
My definition of maining a champion is this. You must be confident in your ability to play that champion in **ANY** matchup. An example is when someone picks Riven and I pick Nasus, having confidence in my ability to win my lane and working hard to do so. You need to have played in different situations and excelled in them **MANY** times. You also have to challenge yourself. Don't stop at better. Be the best.
This revelation came to me when I thought I had reached the maximum skill level I could in my favourite champion atm, only for me to break through and start making plays I never thought I could do. So please dear "mains", especially Riven and Vayne "Mains", stop claiming what you can't back up. Please have **AT LEAST** above a 50% winrate with a champ before you begin to claim that you "Main" them.
Thanks for reading.