Xenia doesn't support mouse input for aiming natively. Most "keyboard pros" use a workaround like GPDSCK or Mouse2Joystick
You can enable and customize basic keyboard inputs by modifying the xenia.config.toml (or xenia-canary.config.toml ) file. xenia emulator keyboard controls
To use a keyboard, you must trick Xenia into thinking your keyboard is an Xbox 360 gamepad. The community-standard tool for this is (Xbox 360 Controller Emulator). Xenia doesn't support mouse input for aiming natively