The Collector characters are characters that are not available anymore at The Shop. You can only get them from other players on the Market, by winning them in ELO Mode, in some Events, or in Collector Coffers (which cost Tokens won from Tournaments).
You can spot a Collector by their name, it ends in "Cr".
Which characters become Collector is decided by the Staff, and you'll be alerted by a home page news one week in advance. At that moment, all existing sales on the Market of the future collectors are cancelled and new sales are blocked for 24 hours (to spread the news). The soon-to-be Collector remain in The Shop's boosters for week and then they become Collectors.