Sonny was the name given by Hange Zoë to a 4m Titan which was captured after the reconquest of Trost District.
Sonny was a small Titan that stood 4 meters high and had short messy blond hair and light blue eyes. It had light peachy-colored skin like the other normal Titans and often had a dazed expression.
Sonny displayed typical traits for a Titan of its class. It exhibited little intelligence, appeared to feel no pain when attacked, and ignored a spear driven into its heart because it had a chance to eat Hange Zoë. It lost activity after an hour of sunlight deprivation and Hange described its personality as "introverted."
Appearance: Sonny was a small Titan that stood 4 meters high and had short messy blond hair and light blue eyes. It had light peachy-colored skin like the other normal Titans and often had a dazed expression.
Abilities: Sonny displayed typical traits for a Titan of its class. It exhibited little intelligence, appeared to feel no pain when attacked, and ignored a spear driven into its heart because it had a chance to eat Hange Zoë.
Sonny in the story: Sonny first appears when the gate of Trost District is broken by the Colossus Titan and numerous Titans invade the district. However, once the breach is plugged by Eren Yeager, human forces are able to retake the district. " They perform a number of experiments on Sonny and Bean, including: testing their response to attempts at communication, injury, and sunlight deprivation.
How Sonny echoes later: In the year 854, Reiner Braun the lone Warrior to return back to their hometown, remembers how it was his idea for him and Annie to kill the captured Titans.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.