I thought it was interesting while reading the two poems to compare Lamia and Geraldine in terms of their relationships with hero-like figures. Hermes and Christabel have interactions with Lamia/Geraldine that are similar in some ways.
Lamia is trapped as a serpent (which in Greek culture is pretty low on the animal totem pole), and Hermes removes the illusion. Through the retelling of her story, Geraldine is 'trapped' as a damsel in distress, but by having Christabel hear her story, the reader realizes that this is an illusion to. The story is not sound. "Five warriors" took her, yes...but where are they? What was a plausible purpose for their leaving, and more importantly, what is her purpose
for staying? There is something Geraldine is not willing to tell Christabel. Geraldine's flawed story suggests the absence of an important detail, or at the very least, that Geradline is not what she seems (which is reinforced later on with the "bad omens" upon Geraldine's arrival).
There are some other slight differences here. Hermes only helps Lamia after she helps him by giving him his water nymph. Geraldine, on the other hand, does not really
give Christabel anything of value, except maybe the opportunity to feel like a moral person by helping her. Whereas Hermes helps Lamia as a
reward for her actions, Christabel takes in Geraldine as an act of kindness.
Lamia is portrayed in more of a submissive role. She immediately wants Lycius and ultimately does not get him; she's punished towards the end of the poem. It is a male figure that gives her her human form (Hermes) and a male figure that ruins her (Apollonius). She has a very reactionary existence.
Gerladine is more conniving. She holds more cards than Christabel (or the reader) is aware of. Both characters play the part of a damsel in distress, but while Lamia goes along with whatever happens to her, Geraldine seems to have an ulterior motive, something that the reader can only approximate since the poem itself was never completed.