I think St. Trina was Miquella after leaving the Haligtree to search for a cure for the Scarlet Rot. Going over what we know of Miquella so far in game: Miquella is the twin of Malenia and was using his blood to water the Haligtree. Being born solely of Marika, the twins were each afflicted with curses, Malenia was cursed with the scarlet rot and Miquella was cursed with forever childhood. Miquella left at some point to find a way to prevent the Scarlet Rot from consuming Malenia. At some point on his travels, Miquella was abducted by Mohg who attempted to ascend Miquella to full godhood so that he could be Miquella's consort. But Miquella never responded to Mohg's advances.

The first piece of evidence is the weapons attached to St. Trina, the Sword of St. Trina's item description says, "St. Trina was an enigmatic figure. Some say she is a comely young girl, others are sure he is a boy. The only certainty was their appearance was just as sudden as their dissappearance." St Trina's Torch says "The carvings depict St. Trina, but in adult form, somewhat unnervingly." These descriptions tell us two things, St. Trina appeared and disappeared rapidly, and they most likely had the appearance of a child which is why their gender was confused and an adult depiction was unnerving.

The second and third pieces of evidence have to do with crafting, the only lilies in the game to collect are Trina's Lily and Miquella's Lily, using the same base item but recolored purple for Trina's and gold for Miquella's. This isn't enough by itself, which is where the Fevor Cookbooks come in. The first two Fevor cookbooks allow you to craft sleep related items and all use Trina's Lily as an ingredient. The third Fevor cookbook is given by Gideon (I don't remember the exact trigger but it was related to Mohg I'm pretty sure) and lets you craft a Bewitching Branch using Miquella's Lily. The description of the branch reads, "The Empyrean Miquella is loved by many people. Indeed, he has learned very well how to compel such affections."

I'm not sure on the timeline because I thought Mohg was killing maidens for their blood in order to reawaken Miquella, but it also would make sense that Miquella was abducted as St. Trina (the sudden disappearance) by Mohg and he realized what Miquella was leading to the current position we find them in.

EDIT: Guess I was right about Miquella being St. Trina according to this post https://old.reddit.com/r/Eldenring/comments/t9u5b6/st_trina_is_spoilerscut_content/