From the Wiki.
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Sage was approached by a scientist named Aristotle Rodor, who had previously been Sage's professor, and Rodor told Sage about an artificial skin called Pseudoderm he co-developed with Dr. Twain. Pseudoderm was intended to work as an applied skin-like bandage with the help of a bonding gas, but had an unforeseen toxicity which was fatal when applied to open wounds. Rodor and Twain agreed to abandon the project and parted ways, but Professor Rodor discovered that Dr. Twain had decided to proceed with an illegal sale of the invention to Third World nations, regardless of the risk to human health.

Sage resolved to stop him but had no way of going after Dr. Twain without exposing himself. Rodor suggested that Sage use a mask made of Pseudoderm to cover his famous features. Armed with information, and more importantly a disguise, Sage eventually caught up with Dr. Twain, stopping the transaction and extracting a confession, and then leaving Twain bound in Pseudoderm in an ironic twist. On television, Sage reported on Dr. Twain's illegal activities.

Sage decided that this new identity, partially inspired by The Spirit, would be useful for future investigations, and partnered with Professor Rodor, who supplied the Pseudoderm and eventually modified the bonding gas to change the color of Sage's hair and clothing. The two men became good friends, with Sage affectionately referring to Rodor as "Tot".