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Combination of design and how he was practically my lucky charm in my first Sapphire run over a decade ago. That same one is waiting to be transferred from my AS cart to Moon, along with three more Swellows bred from it. I arranged for each of them to be used for in-game runs of Pearl, Y, and AS and I was slightly upset that I couldn't do the same for Moon to keep the bloodline going. I'm still going to breed one for the Battle Tree when Pokebank opens but it would've been really nice having it in my first main party.
Pic related, my Swellow from Y.