I am very experienced with Custom Characters, the game I am making is a custom character!
So you would need to do everything that brokenVectors said plus some more to get it fully working:
1: get Roblox's Animation Plugin https://www.roblox.com/library/144373835/Roblox-Animation-Editor
2: create animations with the character(a HumanoidRootPart is required, and needs to be anchored. If you are unaware what the HumanoidRootPart is, it is a transparent part in the torso/lowerTorso that acts like a root. There needs to be a Motor6D connecting the HumanoidRootPart to the Torso/LowerTorso).
3: Press play(do not have StarterCharacter in StarterPlayer) and copy the animation script from your player(R6 Animation script works best)
4: Copy all of the animation id's into the animations parented the script and the animationId's in the script.
5: in Humanoid, set the HipHeight correctly to look best. Find other settings in the humanoid to polish the character's movement.
7: Brag to your friends that you were able to create a successful custom character!
8: Continue working on your game.
9: Stop reading the steps.