Xigbar has always been "that guy" to keep an eye on but not to worry about. But with the big reveal, Riku has learned just how horrible of a person and how dangerous he is. To that end, Riku will always be on guard with Xigbar. He's gotten close enough to touch his heart and try to tear it asunder to reach that darkness inside, and Riku is terrified that he'll do it again. He'll never show that fear and he would, frankly, rather die than be taken over again by the darkness...but until Xigbar and/or MX disappear, he'll never let his guard down or pass them over again.
SPOILERS
Xigbar has always been "that guy" to keep an eye on but not to worry about. But with the big reveal, Riku has learned just how horrible of a person and how dangerous he is. To that end, Riku will always be on guard with Xigbar. He's gotten close enough to touch his heart and try to tear it asunder to reach that darkness inside, and Riku is terrified that he'll do it again. He'll never show that fear and he would, frankly, rather die than be taken over again by the darkness...but until Xigbar and/or MX disappear, he'll never let his guard down or pass them over again.