Sonic

Sonic's body name is "SonicTheHedgehog". If you're interested in modding Sonic, I highly recommend looking into HedgehogUtils which is a mod that handles a lot of Sonic's custom mechanics and other useful things.

Assets

Assets.cs file in the mod contains all of the Sonic meshes, vfx prefabs, and the assetbundle itself with which you can load any other assets you need.

AddScepterToBoostFlash

This method in Sonic's Assets.cs file can be used to add the Scepter Boost's purple electric vfx to the input boost flash prefab. If you're making a custom boost skill in Sonic's utility slot, I recommend making an Ancient Scepter upgrade for the skill for consistency

SkillDefs

All of Sonic's normal skillDefs can be found in the SonicSkillDefs class under the SonicTheHedgehog.Modules.Survivors namespace. If you are going to use these as a starting point for changed versions of the skill, be sure to use UnityEngine.Object.Instantiate(SonicSkillDefs.whateverOriginalSkillDef) so you make a copy of the skillDef and don't change the original

The HedgehogUtils mod also includes a helper method for copying Sonic's Boost skill def. Look at the HedgehogUtils mod for more details.