"They made Scarlet cry. They. Must. Pay." Considering how Pip had only been willing to keep Scarlet around for fanservice and even tried to sell her to pay off a debt at the beginning, this is a huge leap.
Scarlet notices Art's abandoned googly hat and decides it needs a friend, so she leaves it with her googly hat. It's heartwarming in context, see for yourself.
When Kat is depressed that her worst enemy, Hilary, is hotter than she is, Art admits that Hilary is very good-looking, but has "neither the personality nor the charm to back it up". He says it in a way that indirectly but unmistakably declares that Kat does have those qualities and Hilary is very jealous of her for it, which is the real reason behind her hatred.