I like Molly. Her cost of 3 is about average for a Ally, she has some decent soak, and can get you 3-4 named card draws if you are careful about the Assets you use. It's worth making a list of their unique traits, so you know if you want combat, you say "firearm," if you want your Shining Trapezohedron, you say "relic," and so on. The only drawback is that most of the spells have only the trait "spell," so Molly is just going to hand you the first one she finds, which isn't great if you wanted Sixth Sense and you get Spectral Razor instead. However, if you are judicious in your selection of Assets, keeping an eye on traits, you can get pretty close to making Molly a four use No Stone Unturned (5), which is not bad. She also does not get in the way of the "box" signature Showmanship, which puts Dexter in a pretty good place to take both sets of signatures, if you dont't mind a thicker deck.
the only dual trait spells available to Dexter are: Blood Pact (pact), Eye of Chaos (cursed), Seal of the Seventh Sign (ritual), and Song of the Dead (song), none of which particularly scream "Dexter!"