My sad Brothers!AU where Felix was the first Chat Noir and passed on his ring to his younger brother, Adrien, after he died.
Second and third image is of the duo in the past and present. Ladybug started crime fighting before Adrien and was originally partners with Felix. Two years later, Ladybug was introduced to the new Chat Noir.
I imagine Felix to be around 3-4 years older than Adrien since the anime trailer came out 3 years before the 3D show