Thoma remembers how terrified he was when he first fell overboard and landed in Inazuma. He had no mora, no one he knew, he was all alone and terrified...So, he couldn't help but feel sympathy for you when he found you washed up on shore...

You had no memory of who you were or where you were traveling, so he picked you up off the streets and brought you to the Kamisato clan. They recognized you as a member of another clan but decided to let Thoma aid you in regaining your memories anyway...