Alice Bings is the unseen sister of Alec Bings.
All members of the Bings family have powers related to sight (such as Alec's ability to see through things). Hers is the ability to see under things, but due to being still a kid and kids growing down instead of up in the Bings family, she is floating in the air so has trouble using her abilities but what she can't see under, she "overlooks".
Alice's age and appearance are unknown, but Milo is ten, and Alec is said to be around the same age as Milo. Alice is also younger than Alec, making her any age from one to nine. (She's probably not younger than one, as they know what her ability is, meaning she can talk.)