Abigail is the twin sister of Wendy and can be summoned (as a ghost) by using the flower that Wendy is carrying. Her sister will shortly appear.
Abigail follows Wendy around and protects her from dangerous creatures. She will not harm the player or any pet creatures that you have. She will cause damage to monsters but she can also take damager herself and when she takes too much damage, she'll "die" and drop the flower back on the ground. Wendy can pick the flower back up and use it after a short cool down.