For us Japanese, nuance of the is difficult to understand. So I cannot have confidence about my creatures English names.
I try to write down rules about legendary creatures' "the" which I am assuming. Please correct my misunderstandings.
1. Titles which describe the legends literally
Xander, the Collector
Jaxis, the Troublemaker
Errant, Street Artist
I am assuming these below:
Xander is the very great collector and if you simply say "the Collector" at New Capena, it implicitly indicates him.
Samely Jaxis is the outstanding troublemaker among many troublemakers in Capena.
On the other hand, Errant is only a street artist. She is one of many street artists.
But I don't know why Falco is not "the Pactweaver". Probably, Pactweaver is a proper noun and it doesn't require the. But how it became a proper noun? There would be many avens who weave pacts and Falco would be the greatest one among them. If so, I feel he can be called "the Pactweaver" like Xander is the Collector.
2. Titles which describe the legends poetically
For a long time, I thought poetic titles require "the". Wydwen is the Biting Gale, because she is not a gale. Biting Gale is a metaphor for her, so she is not Biting Gale but the Biting Gale.
Ao is not a sky, so he is the Dawn Sky and not Dawn Sky. Doran is not a tower, only it resembles a tower, so he is the Siege Tower. Then here comes Raiyuu, Storm's Edge. He is not an edge but is called Storm's Edge. It confuses me. Why he doesn't have the?
I cannot fluently write English. If you found more appropriate or cooler words for my card name, flavor text or post, I will be glad to hear your advice.
I think it is more a matter of what "sounds exciting" than being consistent. Sometimes the names are written that way to keep card names short; names that are too long could cause printing problems. You can probably use "the" the way you said in 1. for your legendary and it would be communicated okay.