But claiming that being derived from a series makes you a part of it raises a few more questions. After all, Mario's first appearance was in Donkey Kong, and then in Donkey Kong Jr. At the point of his second appearance, you could say that he was a recurring character in the Donkey Kong series (since, with two games now, it could indeed be considered a series). Is he, then, a Donkey Kong character, because he was a character from the Donkey Kong series before getting his own games? If he is no longer a Donkey Kong character, then why would Wario be a Mario character? After all, he's only had a major role in one main-series Mario game, followed by a few other appearances in games with Mario characters (Mario & Wario, Wario's Woods; note that in these games, he's prominently featured in the titles, so these could just as easily be called Wario games) and thereafter he got his own series based on himself. After that, basically his only appearances in Mario games are spinoffs, like Mario Kart. Appearances in spinoffs don't really constitute being a Mario character either; Diddy Kong pretty consistently appears in Mario Kart/Mario Tennis games, but he's very distinctly not a Mario character. Diddy Kong originates from, and otherwise only appears in, Donkey Kong games.
In short, if originating from the Mario series is the reason Wario is a Mario character, then Mario must be a Donkey Kong character, since Donkey Kong was a series in which Mario was a recurring character before he got his own series. If appearing in Mario spinoffs is the reason Wario is a Mario character, then Diddy Kong must be a Mario character, despite originating in and otherwise only appearing in the Donkey Kong series.