Obviously he is the best cat, but let's talk about his significance to Fei Du.
So, it's not subtle that in a way he signifies the part of Fei Du that was mistreated by his father.
Not only does his father expect Fei Du to treat the cat the same way he treats his son, but not killing Luo Yiguo and giving it back to Tao Ran was Fei Du's first rebellion, the realisation that he could maybe one day escape his father's abuse, just like he had helped Luo Yiguo escape.
It's not hard to understand why he would identify with a defenseless little cat that could be crushed so easily by a cruel and violent man like Fei Chengyu.
And then, years later, he learns that Tao Ran entrusted it to Luo Wenzhou. The way he learns it? Seeing the scratches on Luo Wenzhou's hand.
Could you imagine what a man like Fei Chengyu would do to a creature he considers beneath himself, should it have the audacity to scatch him?
And yet, Luo Wenzhou, he might talk big about how he is going to turn the cat into stew, but the fact remains that the cat is now seven years old, fat, the boss of the household, and the consequences of scratching its litter-box attendant? None. He is forgiven, because Luo Wenzhou does not let out his rage on defenseless creatures.
Now the cat, the cat that Fei Du identifies with, has found a home the way Fei Du has always wanted one: a safe space with a person who will take care of him no matter what, who will feed him and not punish him for being the way he is, who will come home every night to keep him company and comfort him.
Fei Du sees the cat he identifies with in the home it feels most comfortable in and realises that could be him.
Apart from the game console, Luo Yiguo is the one big realisation Fei Du has that makes him want to be with Luo Wenzhou. It just takes him a little while to admit that to himself.