This is based on the standard Cat Familiar from DND 5e. The standard stat block would be the same for the familiar itself, below are the quick construction guide for the bat.
Species attributes:
- Speed: 30 ft.
- Darkvision: Accustomed to darkness, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.
- Prowler: You have proficiency in the Stealth skill.
- Scent: You have advantage on Wisdom (Perception) checks that rely on smell.
- First Level Diverse Features: Climber, Lands on Feet
- Recommended Diverse Features for 5th and 10th level: Natural Weapon, Enrage, Exceptional Jumper, Forager, Frightening Roar, Fur Coat, Giant Leap, Hard Biter, Hunter, Leap, Stalker