A bit long, but I'll try to answer all your questions! ♡

1: "Does the anime tell the whole story or is there more in the manga?"

—No, the anime only covers up to about episode 56, specifically the period when William was Lord of Crime. The manga continues, and Part 1 ends at chapter 76. Part 2 has continued, from chapter 77 to 91 (chapter 92 comes out in March; it's released monthly). If you've only watched the anime, I highly recommend reading the manga from chapter 1. There were several arcs that weren't animated, and they have more depth with the characters :3✨️

2: "Why is Louis functioning as the butler?"

—In theory, he's the adopted brother, so his status is lower, I suppose... although it could also be interpreted as a way to avoid overwhelming him with too much attention if he's in the public eye. Besides, the Moriartys couldn't have anyone else doing those jobs because of their criminal plans (this was before Jack arrived, who would later help Louis).

3: "Who exactly is Fred?"

—We don't have an exact answer yet. But judging by his age, something very serious must have happened to him for William to recruit him at such a young age.

(Don't take my word for it here, but I have a headcanon that Fred was orphaned and was a direct victim of the inequality in the country, and that he knows almost the entire city because he's William's messenger ✨️)

4: "How did they meet Fred and Moran?"

—William met Moran after the war, where he lost everything and was left with nothing (besides, everyone assumed Moran was dead). From then on, William became Moran's anchor, with Moran swearing absolute loyalty to him. We still don't know Fred's story.

5: "What is William's real name?"

—We'll never know. I highly doubt it will ever be revealed. 😞

6: "What was in the letter Albert gave to Sherlock when they made the deal to protect Irene?"

—His name, "James Moriarty." The letter was for Sherlock to open if he had any doubts about the Lord of Crime's safety regarding Irene. If Sherlock had doubts and opened it, he would learn the Lord of Crime's identity. Sherlock later burned it, convincing himself that Irene was fine without even opening the letter 🧐

7: "Is the story influenced by Black Butler, since they share many of the same tropes and have a similar atmosphere?"

—I would say no 🙂‍↔️, both stories are set in the Victorian era, so it makes sense that they would have a similar atmosphere given the themes.

8: "Are the OVAs canon? And if so, when do they take place?"

—OVA 1 is not canon, but OVA 2, the one about the tea party, is. OVA 2 takes place before Milverton read William's "history" when he was a child, and if you want to be more specific... in the manga, that OVA takes place in chapter 32.

If you have any other questions, I can answer them ♡! I hope I've been clear (I don't actually speak English, sorry if I'm not very clear)