That’s Sir Lora the Squirrel Knight. His story, and an opportunity to gain him as a companion, is a pre-order bonus for the Definitive Edition of Divinity: Original Sin 2. Folks that already have the regular game on PC will just get him and the Definitive Edition for free when it launches.
This is a companion discussion topic for the original entry at http://www.quartertothree.com/fp/2018/06/21/divinity-original-sin-2-is-going-to-be-a-problem-for-boo-lovers/