Used to be a story pitch for a short animation but better off as a written story I suppose
Loki the cat had always wanted to have a younger brother, but he knows his cat-mom human is not going to adopt any time soon. So when Tony the kitty came into their lives, Loki is not sure how to approach his younger brother. Tony was flighty and somehow super nervous around him. During a really bad storm, Tony was afraid of the lightning and thunder outside, he practically curled up in a ball, shivering. Loki saw this and, with his older feline instinct, went over to Tony to keep him warm in his furs. That was when Tony felt fully secured in his new home.
The two brothers were inseparable, whether in mischief or just chilling out on the sofa, stealing their human's only comfortable chair. One time Loki broke a vase, and their cat-mom had them sit down and asked who broke it; unbelievably, Loki betrayed his brother by putting his paw on his head. The two of them would make it up to each other afterwards, with a good cuddle.
Things are going swimmingly for the brothers, until one day while they were out playing in the sun outside their home, a large tomcat approached them, and he looks like trouble for the brothers. They got into a territorial fight, something that never happened in their home. Their cat-mom, who heard the cats fighting, immediately went out to chase the tomcat. She saw how Loki was wobbly; Loki had tried to protect Tony who was smaller than him and had soothed his brother's shivering body with gentle licks. Since then, the two of them were kept under strict supervision of their cat-mom when they're out playing.
On a stormy night, Loki woke up, noting the warmth coming from his brother who was sleeping next to him gone. He looked around and then heard a small cat scream from the outside; Loki leapt from his bed and ran to the screen door which was ajar. His cat-mom also came out after him and both saw the tomcat running away into the night. Loki saw his brother, a small, little thing, lying on the ground motionless. He nudged him awake to no avail and he heard his cat-mom give out a little sob.
Loki was inconsolable for the next few months, missing his little brother. He even noticed his cat-mom was trying to cheer him up but knew it was futile. Loki was out gazing at the spot where his brother died when he heard his cat-mom return with a basket one afternoon. Inside was a calico kitten with almost the same markings as Tony before. Loki sniffed at the kitten warily, and the kitten mewed at him gently. Loki took the kitten and wrapped him in his fur like he used to with Tony and started grooming him. While the new kitten is not his brother's replacement, he'll do his best to look after him to make it up for Tony.