Not a legendary, but Briar's Pokémon would be absolutely Victini.

Although Briar never captured Victini (and doesn't want to), her friendship with Victini symbolizes her growth in some way.

I don't know how to explain this shortly but Victini symbolizes Briar coming to terms with her feelings regarding the past, the present and the future.

You see, Victini actually relates to Briar a lot. He already has some history with one of Briar's ancestors, and when he met this ancestor, he lived scared of his past—I don't have the details yet of what Victini went through but it has to do with one of the many legendary wars in the Pokémon world. Anyway, the thing is that this ancestor found him and showed him kindness that he hadn't know existed, and hopes to live in the present and look forward to the future.

Following Pokemon's logic, I like to think that Briar is physically similar to this ancestor (like, Ares-Arezu levels of similar; but with a different personality than her ancestor), and immediately stood by her side. It didn't take him long to notice that Briar, just like him, lived in fear of what happened in the past and didn't have a vision for the future, and that blurred her whole way of living; he related to that so he wanted to help her (even if Briar tried to push him aside multiple times). With time, he grew fond of Briar as a person; Victini knew that Briar wasn't the same as her ancestor but he also enjoyed hanging out with her, even if she's difficult and sometimes a little rude. He knows that Briar has something nice inside (very very hidden, but there is something). (:

So yeah, their friendship (that took some time to blossom) symbolizes a story of hope, to say something. Victini wasn't the only one responsible for her character development, there were so many other characters and, of course, herself. But he was still important to her. And needless to say, Victini also helped Briar achieve victory multiple times.

Also I'd like to mention that this friendship and development doesn't necessarily make Briar a good person. Just because she has this positive development doesn't mean it's like redemption to make with her what they did with Lusamine or something like that lol Briar is still Briar (derogatory)

So yeah TL;DR: the answer is Victini