Well, it's obviously Vivillon (implying by 'Kalos' you mean 6th gen instead of pokémon that inhabit Kalos). Never mind. Somebody already beat me to it.
It is a tier in competitive Pokémon battling where you are only allowed to use the first stage of Pokémon who can evolve, and only if they are not good enough for a higher tier. So I will say Fletchling.
1ST CLUE: It's a flying/normal type 2ND CLUE: It's eligible for Little Cup 3RD CLUE: One of its types get replaced when evolving