Okay, a little more backstory! This is why Felix has a Floatzel's soul in addition to his own--the soul is what gives him his Water typing, Floatzel-like personality, and ability to understand and speak to Pokemon. It might need some work, though, so suggestions are appreciated!
So when Felix was about eight, I imagine his parents' lifestyles led him outside a lot. Just outside Floaroma Town is a river where Floatzel and Buizel live, so I'm thinking he befriends a wild Floatzel who he nicknames "Waterfall", because "she's pretty like a waterfall". Waterfall likes Felix, treating him like a Buizel and playing with him as Floatzel are wont to do, and she also introduces Felix to her month-old Buizel pup, whom she allows Felix to nickname Cascade (Felix can't understand Pokémon yet, so Waterfall communicates most of this through gestures).
Then... something bad happens. Maybe Cascade falls into dangerous rapids so Waterfall jumps in after him to shield him with her body, but regardless Felix can't find her at their usual meeting spot. When he eventually does find her downstream, she has horrible gashes all over her body that haunt Felix to this day. Felix had just began learning Pokémon caretaking from his parents, but her injuries were well beyond the capabilities of an experienced vet. In any case, Waterfall dies to Felix and Cascade's grief.
But Waterfall, caring too deeply for Felix and her son, remains after death, her soul bonding with Felix so she could still watch over her son through him. Before she fades into his subconscious, she implores Felix to make him strong, but keep him safe.
After this, Felix realized that he could understand Cascade and his cries for mother, so as he comforts the little Buizel in the Floatzel language, he vows not only to protect Cascade, but to become the best breeder he can, so he doesn't have to watch any more Pokémon die.