How do you be Alucard on castlevania aria of sorrow?

Answer 1

You cannot. There is a ROM hacked version which uses him as the main character, and he is playable in the sequel, Dawn of Sorrow, but unobtainable in regular play of Aria.

Answer 2

To be sure get the good (best) ending by defeating Chaos. After that, you have to start a new game and make your name (the file name) "JULIUS". You will start a new game as Julius Belmont.

Answer 3

You can't without hacking the game. But you can be him in Dawn Of Sorrow if you find him in the Demon Guest House when you are playing in Julius Mode.