Ekko sometimes called Jinx Pow-Pow, as he used to when they were kids, but he knows that Jinx prefers to be called Jinx and not Powder or nicknames that resemble Powder, so every time he said Pow-Pow he would correct himself on the spot by speaking Jinx and therefore he often ended up speaking Pow-Jinx, over time he got used to speaking Pow-Jinx and even Jinx herself got used to and liked the nickname Pow-Jinx as it showed that she could be both and Ekko would still love her.
