Cody from Final Fight wouldn't appear as a playable character in the Street Fighter series until Street Fighter Alpha 3, but made an appearance earlier in Street Fighter Alpha 2 as a background character along with his girlfriend Jessica in the Metro City, USA stage.
For the most part, the character doesn't do much, just standing back and gazing lovingly at his girlfriend. However, as recently highlighted by both TheLegacyOfSega and EventHubs, it's actually possible to break this animation loop to unlock a funny little Easter Egg involving both Cody and Jessica.
To do this, you need to play on the stage as one of the women combatants (such as Chun Li) and walk past Cody. Once you get close to him, you'll then notice that he becomes a bit distracted and starts to pay slightly too much to you causing his girlfriend to get jealous. For a brief moment, Jessica will temporarily lose her cool and clench her fists, before eventually slapping him for looking at other women.
As EventHubs noted, we probably wouldn't see something like this featured in a modern fighting game today, but it's a funny, little bonus that does a lot to add a bit of personality to the individual characters and the stage itself.
When Cody next appeared in Street Fighter Alpha 3, unsurprisingly, he and Jessica had broken up, with the two eventually going their separate ways; Jessica heads off to Europe, and Cody ends up in Prison after getting in too many street fights.