From the author's README on Thunderstore

Sonic Costume

Sonic Costume as a full-body costume for skate., running on ReSkate.

Shows up in Apparel → Costumes as Sonic.

Version 1.0.0

Full-detail model with source textures.
Rigged to Isaac with custom weight transfers.
UV maps and material slots split and mapped precisely to fit ReSkate Studio's container layout.

Install

1. Copy this folder into Skate\Mods.

2. Start the game through the ReSkate launcher and enable Sonic Costume in the mod list if it isn't already.

3. In game, open Apparel → Costumes and equip Sonic Sonic.

Built for ReSkate.

Notes

Personal use only. The model and textures were converted locally. Sonic and its content belong to SEGA. This mod is not affiliated with or endorsed by SEGA, Electronic Arts, Full Circle, or the ReSkate project.
Built with ReSkate Studio (custom cosmetic from the Isaac template) and Blender.
I still need to adjust the mesh so please don't hate me :X

Install Sonic Costume

You need ReSkate and your own copy of skate. on Steam.

Open ReSkateLauncher.exe and go to MODS.
Search for Sonic Costume and click install. The launcher also installs anything it depends on.
Press PLAY. In game, the MODS tab of the ReSkate menu (Insert) turns it on and off.

Or install it by hand: download the .zip and drop it on the launcher window, or unzip it into its own folder in Mods\ beside Skate.exe.

Maps show up in the ReSkate menu's level list. To join a server running a custom map, you need the same map installed. If the game won't load after installing, see known issues.

skatemods doesn't host or review this mod. The download comes straight from Thunderstore. Only install mods you trust, because scripts can run code.