In the magazine the lead character designer stated that Raboot was Sobble's "アニキ" otherwise known as his Aniki, it was also confirmed by another Japanese translator in the linked tweet. If it was a female or a big sister they would have used Aneki instead. So yea, the rabbit is a dude.
兄貴(アニキ / aniki)是來自日本的漢字詞語。日語原意是對兄長或者男性長輩的尊稱,與漢語中的「兄台」、「大哥」、「阿兄」近義,也常是對江湖中人的尊稱。