Hourman

Hourman was a member of the Justice League after the Thanagarian invasion.

Background information
The action figure of this character, released by Mattel in the Justice League Unlimited line has confirmed that this is indeed Hourman.

In the comics, there were two Hourmen: scientist Rex Tyler and his son, Rick. Rex invented a vitamin called Miraclo, which gave him super strength and speed that lasted one hour, but the vitamin proved highly addictive. Eventually, this was worked around.

The DCAU's Hourman appears to be based on Rick Tyler, current member of the JSA. He found a way to inject the miraclo into his veins by pressing a button on his glove, the same way Hourman does in "Panic in the Sky"