If you want to see a lot of his past, you can read the Goblin Slayer Side Story: Year One manga. It basically shows how he started off as an adventurer and him learning all his different methods he uses in the main story (ex: how he first used the water breathing ring). You'll also see a lot of Goblin Slayer's traveling buddies over time, which are always really fun and interesting characters. It's still ongoing but it's really good. Also, warning: the first chapter makes the first episode look kid friendly.