Some abilities, such as Renekton's slice and dice, have obvious ability icons, and Pix is one of them. After all, the silhouette of the being in question is on the ability icon.
In today's challenge , the ability of the day is: "Pix, Faerie Companion". At first glance, we can recognize the champion using said ability.
The answer to which LoL champion uses Pix, Faerie Companion is Lulu. The icon itself shows the being we actually see in-game.
In the lore, Pix is a being who uses Lulu to is own benefit, without Lulu even knowing it. Its a Faerie through and through, and even a yordle can't deal with such nuisances.
Pix has chosen Lulu because of her naivete and magical potential, showing her the Faerie world, and now manipulates her to roam all over Runeterra, and, of course, take it with her, as it can't do so on its own.
The downside is that, until Pix is satisfied, it has barred Lulu from returning to both the Faerie world she likes so much, as well as her home, Bandle city, where all of her friends are.
And, in her ignorance, lulu can't even recognize how malicious Pix's actions and intentions are.