And where can I meet these characters or is it not added to the game?
Those characters are not currently in the game. They will be added in a later update.
I noticed that the 2 new characters, the centaur and the mermaid, both have weapons, and the other 5 new characters have no weapons?
Those characters may not be bosses or enemies but instead might be an NPC that Nene has to help in story/quest related events, similar to Bell.
Yes, there will be additional stories in the future that will help some of those characters😙