Abigail is Wendy's sister and a non-playable character in the game Don't Starve though what happened to her, we don't know. She can only be summoned by using a flower that appears in Wendy's inventory at the start of the game. Once summoned, she will protect Wendy from non-pet creatures in the world. She will follow Wendy around and won't harm Wendy. At night, Abigail provides a small amount of light and does more damage to enemies.
Abigail can heal damage done to her but if she takes too much damage, she will "die". When she does, she'll drop the flower back on the ground and after a cooldown period, Wendy will be able to summon her again.