Final answer:
Rainbow Baby is the familiar in The Binding of Isaac that has the described attributes and can be obtained by destroying skulls in the Depths and Necropolis floors.
Step-by-step explanation:
The familiar in The Binding of Isaac that follows Isaac, shoots spectral tears dealing 3.5 damage, and fires tears at a rate of 1 tear per second is Rainbow Baby.
Rainbow Baby is obtained by destroying skulls in the Depths and Necropolis floors.