tl;dr no one understands the system but we think winning a lot increases rank and you need to be good and helping your team win, as in outplaying everyone matters a lot. Also doing well 10 matches and mediocre the 11th doesn't mean you won't get a full rank.
You're better with characters x or y so you get more rank with them.
You're better with characters x or y so you get more rank with them.