Here, You can find spritesheets and an overview of Julius' abilities in the games he appears in. While I love playing as him, I'm no expert on the fine details of character stats, so I will also link some resources that explain that better for those who are interested. I will also write a bit about Julius mode speedruns in Aria of Sorrow.
After finishing the main game, it is possible to play as Julius in both Aria of Sorrow and Dawn of Sorrow. This offers a less linear gameplay experience due to Julius' larger mobility, and also a different challenge, because Julius cannot use items and equipment. Listed here, you will find links to in-depth guides about playing Julius mode and his stats.
Aria of Sorrow:
Julius Belmont bossfight guide
Dawn of Sorrow:
While Julius mode in DoS has an amazing story, in AoS, Julius mode is fun to play but lacks plot. Linked here is a rom hack that adds plot to the AoS Julius mode, and also allows him to use items and equipment. The game is somewhat buggy but amazing gameplay-wise and of course it has a compelling story. You can download it Here.
Although I've played through Julius mode in AoS many times, I am not a speedrunner, and I don't aspire to be. However, I do love watching Julius mode speedruns, especially tool-assisted speedruns where he's bouncing around the screen like crazy. Those truly tickle my brain. His ULEULEULEULEULE sound has become a bit of a meme by now. I love that times are still being improved, even now, 20+ years after the game came out. I'm not even sure what the record is right now, but I believe it's around 5 minutes or even lower for TAS. Below, you will find some links where you can learn more about Julius mode speedruns.
Fighting Julius and Chaos in Julius mode
Julius mode TAS (ULEULEULEULEULE)
Julius mode boss rush with Graham 2 skip