Skateboard guy is seen in the video showing initial clash between armed BLM sympathizers and unarmed BLM protesters–the video where a white guy in a red shirt bounces around shouting "shoot me nïgga".
It also appears that Skateboard guy is one of several in a group that attacks Kyle Rittenhouse when Rittenhouse trips while being chased by a mob. Rittenhouse is struck in the head with a skateboard, before shooting the assailant.